You could do a remap but it’s going to be hard to differentiate between mentions for the “user in particular” and mentions for the “user who talk about X” 
Depending on how popular that user was, you could manually find all the posts with a mention to that user and edit them to use a mention to the group when they make sense.
Alternatively, you could rename mentions “as you go” (aka. whenever you or a staff member if you have any come across one of those mentions).