Lock for Tickets

Are you sure it is what you expect ?
If locked, ticket will not be updatable unless unlocked.
So resource working on ticket will find it hard to deal with unlocking / locking to trace all changes on tickets.

Wouldn't it be better (more complex I guess but much better) to be able to "fix scope" of a version :
=> items tagged with this target version not able to chenge version
=> no new item allowed on this version
With some kind of explicit message : "this version scope is fixed, cannot move or add any item to this version"

What do you mean by "scope of the project"