I realize that the tag renaming functionality has existed in its present form for quite some time and, as far as I know, no one has a problem using it, but I wanted to throw this out there in case anyone else is a bit tripped up about the difference between renaming and merging tags.
The helper text reads:
If the Remove Tag exists, it will be merged into the Master Tag, and then deleted; the Master Tag will exist after the merge.
If the Remove Tag does not exist, the Master Tag will be renamed to the Remove Tag (which will exist after the merge).
My problem with this is that which tag exists after the merge/rename depends on whether or not the remove tag exists. To me, it makes much more sense to have the master tag always be the tag that remains after the operation is completed. If the master tag exists, the remove tag is merged in. If the master tag doesn't exist, the remove tag is renamed. In essence it becomes the same operation either way - the remove tag is replaced by the master tag whether or not the master tag exists.
Of course, it's possible that if the functionality is switched it would trip up everyone that's already used to the way it works now, but I have a sneaking suspicion that most people read the helper text carefully each and every time to make sure they don't nix the wrong tag.