User Post Restoration

I'm hoping you'll guys will be able to help me... My username got deleted on my forums, no problem I figured so I'd just create a new user with the same name... Well, everything went okay except all of my old posts have turned into, "guest" posts... I'm figuring a simple query in phpmyadmin would fix it. So in essence, I'd want to do a IF Username = Ian Than userid = 16... It'd be in a different syntax though.

[Ian]

Hmm, anyone know how?

[Admin]

UPDATE post SET userid=[high]yourNewID[/high] WHERE username="Ian";
UPDATE thread SET postuserid=[high]yourNewID[/high] WHERE postusername="Ian";