diff --git a/index.js b/index.js
index da168f01..3e9e24e5 100644
--- a/index.js
+++ b/index.js
@@ -20,6 +20,7 @@ const {
   ASSERTION,
   BAD_ARGUMENT,
   SESSION_CLOSED,
+  SESSION_MOVED,
   SESSION_NOT_WRITABLE,
   SNAPSHOT_NOT_AVAILABLE,
   DECODING_ERROR
@@ -248,8 +249,11 @@ class Hypercore extends EventEmitter {
     if (!this.keyPair) this.keyPair = parent.keyPair
     this.writable = this._isWritable()
 
-    if (parent.state) {
-      this.state = this.draft ? parent.state.memoryOverlay() : this.snapshotted ? parent.state.snapshot() : parent.state.ref()
+    const s = parent.state
+
+    if (s) {
+      const shouldSnapshot = this.snapshotted && !s.isSnapshot()
+      this.state = this.draft ? s.memoryOverlay() : shouldSnapshot ? s.snapshot() : s.ref()
     }
 
     if (this.snapshotted && this.core && !this._snapshot) this._updateSnapshot()
@@ -329,7 +333,7 @@ class Hypercore extends EventEmitter {
     }
 
     if (opts.parent) {
-      if (opts.parent.state === null) await opts.parent.ready()
+      if (opts.parent._stateIndex === -1) await opts.parent.ready()
       this._setupSession(opts.parent)
     }
 
@@ -564,6 +568,28 @@ class Hypercore extends EventEmitter {
     return p
   }
 
+  transferSession (core) {
+    // todo: validate we can move
+
+    if (this.weak === false) {
+      this.core.activeSessions--
+      core.activeSessions++
+    }
+
+    if (this._monitorIndex >= 0) {
+      this.core.removeMonitor(this)
+      core.addMonitor(this)
+    }
+
+    const old = this.core
+
+    this.core = core
+
+    old.replicator.clearRequests(this.activeRequests, SESSION_MOVED())
+
+    this.emit('migrate', this.key)
+  }
+
   createTreeBatch () {
     return this.state.tree.batch()
   }
@@ -609,7 +635,12 @@ class Hypercore extends EventEmitter {
       const activeRequests = (opts && opts.activeRequests) || this.activeRequests
       const req = this.core.replicator.addUpgrade(activeRequests)
 
-      upgraded = await req.promise
+      try {
+        upgraded = await req.promise
+      } catch (err) {
+        if (isSessionMoved(err)) return this.update(opts)
+        throw err
+      }
     }
 
     if (!upgraded) return false
@@ -636,7 +667,12 @@ class Hypercore extends EventEmitter {
     const timeout = opts && opts.timeout !== undefined ? opts.timeout : this.timeout
     if (timeout) req.context.setTimeout(req, timeout)
 
-    return req.promise
+    try {
+      return await req.promise
+    } catch (err) {
+      if (isSessionMoved(err)) return this.seek(bytes, opts)
+      throw err
+    }
   }
 
   async has (start, end = start + 1) {
@@ -758,9 +794,16 @@ class Hypercore extends EventEmitter {
     const timeout = opts && opts.timeout !== undefined ? opts.timeout : this.timeout
     if (timeout) req.context.setTimeout(req, timeout)
 
-    const replicatedBlock = await req.promise
-    if (this._snapshot !== null) checkSnapshot(this, index)
+    let replicatedBlock = null
+
+    try {
+      replicatedBlock = await req.promise
+    } catch (err) {
+      if (isSessionMoved(err)) return this._get(index, opts)
+      throw err
+    }
 
+    if (this._snapshot !== null) checkSnapshot(this, index)
     return maybeUnslab(replicatedBlock)
   }
 
@@ -790,19 +833,7 @@ class Hypercore extends EventEmitter {
   }
 
   download (range) {
-    const req = this._download(range)
-
-    // do not crash in the background...
-    req.catch(safetyCatch)
-
-    return new Download(req)
-  }
-
-  async _download (range) {
-    if (this.opened === false) await this.opening
-
-    const activeRequests = (range && range.activeRequests) || this.activeRequests
-    return this.core.replicator.addRange(activeRequests, range)
+    return new Download(this, range)
   }
 
   // TODO: get rid of this / deprecate it?
@@ -841,9 +872,9 @@ class Hypercore extends EventEmitter {
     const defaultKeyPair = this.state.name === null ? this.keyPair : null
 
     const { keyPair = defaultKeyPair, signature = null } = opts
-    const writable = !this._readonly && !!(signature || (keyPair && keyPair.secretKey))
+    const writable = !!this.draft || !isDefault || !!signature || !!(keyPair && keyPair.secretKey)
 
-    if (isDefault && writable === false) throw SESSION_NOT_WRITABLE()
+    if (this._readonly || writable === false) throw SESSION_NOT_WRITABLE()
 
     blocks = Array.isArray(blocks) ? blocks : [blocks]
 
@@ -1031,3 +1062,7 @@ function maybeAddMonitor (name) {
     this.core.addMonitor(this)
   }
 }
+
+function isSessionMoved (err) {
+  return err.code === 'SESSION_MOVED'
+}
diff --git a/lib/core.js b/lib/core.js
index 67def666..08a0a030 100644
--- a/lib/core.js
+++ b/lib/core.js
@@ -268,8 +268,6 @@ module.exports = class Core {
     this.verifier = verifier
     this.state = new SessionState(this, storage, this.blocks, tree, null)
 
-    this.state.ref()
-
     if (this.key === null) this.key = this.header.key
     if (this.discoveryKey === null) this.discoveryKey = crypto.discoveryKey(this.key)
     if (this.id === null) this.id = z32.encode(this.key)
@@ -402,7 +400,7 @@ module.exports = class Core {
         signature = this.verifier.sign(state.tree.batch(), keyPair)
       }
 
-      const tree = await this.state._overwrite(state, length, treeLength, signature)
+      const tree = await this.state._overwrite(state, length, treeLength, signature, false)
 
       // gc blocks from source
       if (treeLength < length) {
diff --git a/lib/download.js b/lib/download.js
index 8aed76fd..6c88505f 100644
--- a/lib/download.js
+++ b/lib/download.js
@@ -1,10 +1,39 @@
 module.exports = class Download {
-  constructor (req) {
-    this.req = req
+  constructor (session, range) {
+    this.session = session
+    this.range = range
+    this.request = null
+    this.opened = false
+    this.opening = this._open()
+    this.opening.catch(noop)
+  }
+
+  ready () {
+    return this.opening
+  }
+
+  async _open () {
+    if (this.session.opened === false) await this.session.opening
+    this._download()
+    this.opened = true
   }
 
   async done () {
-    return (await this.req).promise
+    await this.ready()
+
+    try {
+      return await this.request.promise
+    } catch (err) {
+      if (isSessionMoved(err)) return this._download()
+      throw err
+    }
+  }
+
+  _download () {
+    const activeRequests = (this.range && this.range.activeRequests) || this.session.activeRequests
+    this.request = this.session.core.replicator.addRange(activeRequests, this.range)
+    this.request.promise.catch(noop)
+    return this.request.promise
   }
 
   /**
@@ -15,8 +44,17 @@ module.exports = class Download {
   }
 
   destroy () {
-    this.req.then(req => req.context && req.context.detach(req), noop)
+    this._destroyBackground().catch(noop)
+  }
+
+  async _destroyBackground () {
+    if (this.opened === false) await this.ready()
+    if (this.request.context) this.request.context.detach(this.request)
   }
 }
 
 function noop () {}
+
+function isSessionMoved (err) {
+  return err.code === 'SESSION_MOVED'
+}
diff --git a/lib/merkle-tree.js b/lib/merkle-tree.js
index a591339f..9728133a 100644
--- a/lib/merkle-tree.js
+++ b/lib/merkle-tree.js
@@ -516,14 +516,6 @@ module.exports = class MerkleTree {
     return new MerkleTreeBatch(this)
   }
 
-  snapshot () {
-    return {
-      length: this.length,
-      byteLength: this.byteLength,
-      fork: this.fork
-    }
-  }
-
   async restoreBatch (length) {
     const batch = new MerkleTreeBatch(this)
     if (length === this.length) return batch
diff --git a/lib/session-state.js b/lib/session-state.js
index da92dbc3..35a56c79 100644
--- a/lib/session-state.js
+++ b/lib/session-state.js
@@ -31,6 +31,8 @@ module.exports = class SessionState {
     this._onflush = null
     this._flushing = null
     this._activeBatch = null
+
+    this.ref()
   }
 
   isSnapshot () {
@@ -90,19 +92,7 @@ module.exports = class SessionState {
       this.core,
       this.storage.snapshot(),
       this.blocks,
-      this.tree.snapshot(),
-      this.name
-    )
-
-    return s
-  }
-
-  snapshotFrom (snap) {
-    const s = new SessionState(
-      this.core,
-      snap.storage,
-      this.blocks,
-      snap.tree,
+      this.tree.clone(),
       this.name
     )
 
@@ -304,8 +294,6 @@ module.exports = class SessionState {
       await storeBitfieldRange(this.storage, storage, batch.ancestors, batch.treeLength, false)
     }
 
-    if (dependency) storage.setDataDependency(dependency)
-
     return { dependency, tree, treeUpdate }
   }
 
@@ -413,7 +401,7 @@ module.exports = class SessionState {
     }
   }
 
-  async _overwrite (source, length, treeLength, signature) {
+  async _overwrite (source, length, treeLength, signature, isDependent) {
     const blockPromises = []
     const treePromises = []
     const rootPromises = []
@@ -500,8 +488,12 @@ module.exports = class SessionState {
       this.tree.setRoots(roots, signature)
     }
 
+    const dependency = isDependent ? updateDependency(this, length) : null
+
     await this.flushWriteBatch(writer)
 
+    if (dependency) this.storage.updateDependencies(dependency.length)
+
     return tree
   }
 
@@ -513,7 +505,7 @@ module.exports = class SessionState {
     try {
       const origLength = this.tree.length
 
-      const tree = await this._overwrite(state, length, treeLength, null)
+      const tree = await this._overwrite(state, length, treeLength, null, state === this.core.state)
 
       const bitfield = { start: treeLength, length: tree.length - treeLength, drop: false }
 
@@ -545,6 +537,32 @@ module.exports = class SessionState {
     return head
   }
 
+  async moveTo (core) {
+    const state = core.state
+
+    if (state.storage && (await state.storage.openBatch(this.name)) !== null) {
+      throw STORAGE_CONFLICT('Batch has already been created')
+    }
+
+    const head = this.core.sessionStates.pop()
+    if (head !== this) this.core.sessionStates[(head.index = this.index)] = head
+
+    this.core = core
+    this.index = this.core.sessionStates.push(this) - 1
+
+    if (!this.isSnapshot()) {
+      const treeInfo = await state._getTreeHeadAt(this.tree.length)
+      const prologue = state.tree.prologue
+
+      // todo: validate treeInfo
+
+      this.storage = await state.storage.registerBatch(this.name, treeInfo)
+      this.tree = await MerkleTree.open(this.storage, treeInfo.length, { prologue })
+    }
+
+    for (const s of this.sessions) s.transferSession(this.core)
+  }
+
   async createSession (name, length, overwrite, draft) {
     let storage = null
     let treeInfo = null
diff --git a/test/move-to.js b/test/move-to.js
new file mode 100644
index 00000000..2a96d411
--- /dev/null
+++ b/test/move-to.js
@@ -0,0 +1,106 @@
+const test = require('brittle')
+const b4a = require('b4a')
+const crypto = require('hypercore-crypto')
+const { create } = require('./helpers')
+
+test('move - basic', async function (t) {
+  t.plan(9)
+
+  const core = await create(t)
+
+  const sess = core.session({ name: 'session' })
+
+  await sess.append('1')
+  await sess.append('2')
+  await sess.append('3')
+
+  await core.core.commit(sess.state)
+
+  t.is(core.length, 3)
+  t.is(sess.length, 3)
+
+  const keyPair = crypto.keyPair()
+
+  const manifest = {
+    prologue: {
+      length: core.length,
+      hash: core.state.tree.hash()
+    },
+    signers: [{
+      publicKey: keyPair.publicKey
+    }]
+  }
+
+  const core2 = await create(t, { manifest, keyPair })
+  await core2.core.copyPrologue(core.state)
+
+  t.is(core2.length, 3)
+
+  sess.once('migrate', key => { t.alike(key, core2.key) })
+
+  await sess.state.moveTo(core2.core)
+  await sess.append('4')
+
+  await core2.core.commit(sess.state)
+
+  t.alike(await sess.get(0), b4a.from('1'))
+  t.alike(await sess.get(1), b4a.from('2'))
+  t.alike(await sess.get(2), b4a.from('3'))
+  t.alike(await sess.get(3), b4a.from('4'))
+
+  t.alike(await core2.get(3), b4a.from('4'))
+
+  await core.close()
+  await core2.close()
+  await sess.close()
+})
+
+test('move - snapshots', async function (t) {
+  const core = await create(t)
+
+  await core.append('hello')
+  await core.append('world')
+  await core.append('again')
+
+  const sess = core.session({ name: 'snapshot' })
+
+  const snap = sess.snapshot()
+  await snap.ready()
+
+  await sess.close()
+  await core.truncate(1)
+
+  await core.append('break')
+
+  t.is(snap.length, 3)
+  t.is(core.length, 2)
+
+  const keyPair = crypto.keyPair()
+
+  const manifest = {
+    prologue: {
+      length: core.length,
+      hash: core.state.tree.hash()
+    },
+    signers: [{
+      publicKey: keyPair.publicKey
+    }]
+  }
+
+  const core2 = await create(t, { manifest, keyPair })
+  await core2.core.copyPrologue(core.state)
+
+  t.is(core2.length, 2)
+
+  await snap.state.moveTo(core2.core)
+
+  t.is(snap.length, 3)
+
+  t.alike(await snap.get(0), b4a.from('hello'))
+  t.alike(await snap.get(1), b4a.from('world'))
+  t.alike(await snap.get(2), b4a.from('again'))
+
+  await snap.close()
+  await core.close()
+  await core2.close()
+})
diff --git a/test/snapshots.js b/test/snapshots.js
index 05b239e8..c02e0243 100644
--- a/test/snapshots.js
+++ b/test/snapshots.js
@@ -158,7 +158,11 @@ test('snapshot over named batch persists after truncate', async function (t) {
   await core.append('block #2.0')
 
   const session = core.session({ name: 'session' })
+
   const snapshot = session.snapshot({ valueEncoding: 'utf-8' })
+  await snapshot.ready()
+
+  await session.close()
 
   t.is(snapshot.length, 3)