by Ken Task.
You could test by editing one of the users in users.xml from an extracted 2.7 backup. User would be known. Re-package the backup. Restore to the 3.6.x. See if the user you edited in users.xml appears as they should.
Are all the accounts restored already to the 3.6.x instance?
Seems to me since this is really a migration by backup/restore one would not have any users in the 3.6.x cept admin and guest. Restore of just one course and one could have less complicated DB queries of mdl_user table to change auth, username columns of mdl_user table - that's just a thought.
'SoS', Ken