So, #Smithereen no longer requires a username when signing up. You instead get assigned one automatically, of the form "idXXX" where XXX is the ID for your user record. You can change it later in settings if you wish. Current Mastodon versions handle this change no problem.
The username is only visible in two places:
- The URL of your profile
- When someone mentions you
So it's not very important. Definitely not important enough to be a required field when you register.