Does that mean IPB uses usernames to track users instead of, say, an immutable ID number?
That's...interesting.
No, it's an issue with the migration process specifically. Login procedures are changing, and the way it'll work is that display names will be used for login as well.
Obviously, there's an issue if your display name is something crazy.
Ahh, I see. That seems reasonable, since your display name might be permanently changed to something other than your account name.