Change the Id field of the user

We discovered a rare case in RPAS where the user had a username as nmlm11 and the Id field as nmlm110. The Id field was different from the username. Now the user has requested to change the Id field to the same as the username field in RPAS. I would like to understand the below things:

1) How was the Id field different from the username field?

2) How do we change the Id field to the same as the username field?

3) Will there be any impact on the other information that the user profile contains?

4) Will the user be prompted for the change of password after the change?