Obviously I would need the schema to Elitefitness's Database, but the problem should be fixable in 2 seconds...
A user should be unique in the database correct?
Why can't you simply go into your database
Update ElitefitnessDB.Posts
set userhandle = 'Gambino'
Where userhandle = 'Ludendorf'
Update EliteFitnessDB.Users
set userhandle = 'Gambino'
Where userhandle = 'Ludendorf'
*Poof*