This is due to two specific changes from the 1.0 engine to the 2.0 engine.
The first is that the minimum length of display names on the SE 2.0 engine is now three characters, not two. Names shorter than two characters were normalized in the pattern of user{user_id}
. This is something they can change by editing their profile.
The second is that the engine no longer infers a username from OpenID. If you want anything other than the standard normalized user{user_id}
, you need to deliberately set it. Accounts where one had been inferred were automatically changed to that effect. Again, the owners of those accounts are free to make the change.
Note - the system will only allow you to change your username once every 30 days. If someone affected by this were to make a mistake while changing their name back and need help correcting it, we're happy to do so.
I think this is the only other account related quirk as a direct result of the transition, but it's possible that others may come up. If they do, please continue to raise them here so we get them documented and folks encountering them find help :)