Changing taxonomy term id looses existing content values

[laulaz]

With the new ability to change terms identifiers (see #72 & #113), we can now change the identifier of a term, even if it is already used on a content (value selected on the behavior).
As there is no migration code in the pull request, the existing values are then "lost".

IMHO, we should :

• migrate the existing values used (if their identifier changed)
• prevent user to change terms identifiers that are already used
• or at least warn the user in the control panel

@Petchesi-Iulian @ale-rt @alecghica @avoinea what do you think of this ?

[alecghica]

@laulaz make perfect sense to me!

We can take care of this but it will take some time to be implemented as it's low prio for us at least.
Do you plan to implement it? If so we won't put it on our radar. Just let me know.