Good question. To the best of my knowledge, no syncing is done between MNet servers except when user from Moodle A attempts to login to Moodle B.
So a Moodle A user (hpotter) is a unique user. If the user name in Moodle A is changed, then Moodle B wouldn't be advised of any change and probably try to register the user as a new user. EMail address changed as well?
One way to find out for sure: create a user hpotter in Moodle A, enrol that user in a Moodle B course via MNet. Make sure hpotter does participate (post to forum/take quiz, etc.). Change hpotter user name in Moodle A and see what happens! ;)
If the system won't do it, the 'backdoor' *might be* to edit mdl_user table on Moodle B to match changes made on Moodle A.
'spirit of sharing', Ken