This enhancement request is related to 13507 but is not the same. The request here is to implement a mechanism in which a trusted user (steward or some global renamer group if one is created) to globally rename a user that has already been unified. The details would need to be worked out, but the idea is that the system would check to see if the target name has been taken on any wikis, and if it has, drop the global association on those wikis (if prompted to do so) or abandon the request (if the prompt was answered negatively)... users would then have to seek local rename on those wikis (perhaps via usurp). (once renamed the behaviour we see now where the newly renamed account can be properly associated with the global one (which is awesome) would allow reassociation).
We stewards are seeing an increasing number of requests for rename. The current way to do it, I think, requires un SULing, and local renames on every wiki where there is an account, and then re SULing.