Rename User IDs for existing visits

Hi,
Couple of quick questions…
Background: we historically had users real names captured as the User ID field. We’ve since moved to a different pattern and but have a bunch of existing User IDs which still have these real names. Ideally, we’d like to preserve these visits but rename the User IDs

Is there anyway to change existing User IDs from the UI?
Is there anyway to merge two Users IDs?

Thanks

Neil