Details
Description
The current implementation of $rename prohibits any rename where the source and destination are along the same path: e.g., {$rename: {'a':'a.b'}} or {$rename: {'a.b':'a'}}. There is a TODO noting that it would be possible to relax this restriction.
If we do want to make that change, now would be a good time, as we are making backwards-incompatible changes to update functionality.