You can via the DB.... but honestly, I'd simply temporarily give them a special group with unlimited edit time and tell them to knock themselves out reviewing their posts and making the changes. Give them a few days then disable that special group.
Or you could edit the database, but realize that doing it that way unless you filter based upon specific user any reference to that name, so you have to be more careful.
Cheers. I told them to add a post saying x name is now called y name and start a new thread about the name change. It seemed a bit much editing all old threads when others had replied using the old pet name (and it was in quotes etc).