Having issues upgrading my account to the collector's edition digital version, keeps saying "The supplied serial code is valid, but it can not be used to create a new account and it can not be applied to any accounts you already have". Anyone know how to fix this?

Sounds like the server is confused or you're doing something wrong.
open up your NCsoft account, log in and click the add code thing. if that doesn't work, i'd call them up.
Chances are the servers are messed up due to a large load of people trying to access the same stuff at once

Lol I'm dumb, my acct was still pre-order status because I didn't input my code from the actual game. It's been a long day...