What happens if two people have similar usernames and one of them becomes a staff member?
Does the other one get banned?
A chance to change their username?
Nothing happens?

No ban would take place. Nothing would probably happen, at the most they would be offered a free name change. Even then, that's a stretch. Similar usernames aren't the issue, it's when someone with a similar name tries to act like they're part of staff but they're not.