LOL! It'll be all good as long as you backup right before. Then even if it all goes wrong you can always restore from backup when the "fun" is over.
Although, couldn't you just use replacements do create a similar effect? Just make a replacement set where "Username1" changes to "Username6" and so on, and when #1 logs in, it will say "Welcome Username6" and all their posts that day will have the names all switched. Then simply delete the replacements and there's no mess to clean up.