With the move to the new forum format here there has been a slight change in the way usernames work. All your previous account data migrated in without a problem.
The only change is underscores and spacing. If your username previously had a “space” in it. That space has been replaced by an “underscore.”
Old username: Mike Smith
Old username: Mike_Smith
This was a necessary move for usernames to comply with the new systems @ tagging system.