Hi, Username just can modified one time by rule, of osclass core.
You said, I can rid of it. I assume when you try to change username, and not works anymore. Just happens with a particular user or for all users?
Maybe you saved with default number without changes. I never tryed before, but maybe is possible save with same value, and assume it already saved one time, and no changes anymore to user name in particular.
Check that first with other users (test users).