Instead of changing the UserID to the account which could be intensive, why not just have the FORUM DISPLAY NAME be the same as the CHARACTER NAME that you select?

That way it's the most current and this "feature" isn't new, take a look at the WoW forums that pull character information and allow people to post as the character rather than their login ID.

We're already halfway there with the character mini-bios with the avatar, why not the forum name too?