Found by using the "Poor Man's Devtracker" - that is, clicking Cordovan's name and looking at his recent posts:

Quote Originally Posted by Cordovan View Post
For folks who are in this situation (meaning you are prompted to choose a new display name despite having a community account previously, and are not able to choose your previous community name): Send in a ticket to Community/Forum Support through We're gathering up everyone's information while we work on a fix for this. Once we have a more solid resolution plan, we'll be messaging you (probably both publicly and through the ticketing system.) What's happened is that your Community Account information has been made inaccessible due to a forum migration issue, and it's something we're working to fix as soon as possible.
In summary, it appears they don't have a fix yet, but are researching it. So they can't fix anyone's account yet I suppose, and we have no idea how long it will take. We don't even know when we'll get another update on the matter.