I mean it'd theoretically let you just try to get a name you want with the new username system well before you would otherwise. If they aren't completely committed to rolling this out in the dumbest possible way.
I don't know any good reason why they wouldn't let new accounts join in with the new usernames, but I also don't know why they would ever think any of this was a good idea.
I think they may soft reserve names similar to the one you currently have before your numerical code, so the first person with that handle gets first dibs? I may be wrong tho