Hmm, that might be a tough one to solve. What happens during the upgrade is the sender and receiver used to be the actual usernames but were converted to userids. You might try looking at a couple of old PMs in the db and see what the fields are set to. THis would give us an idea if there is anything we can do.
SELECT M_Uid,M_Sender FROM w3t_Messages ORDER BY M_Number LIMIT 5
That will give you the uid and sender of the first 5 messages in the system.
