by Ken Task.
Follow up ... there is a plugin to 'merge' accounts.
https://moodle.org/plugins/tool_mergeusers
Does show compat with 3.6.x
Have never used it so don't know how well it performs or if it will work as you would want. Got a test setup? ... clone of the 3.6 where you could tinker all you like!
A few years ago got involved with a site that was changing auth from manual to a backended samil2 box. Site admin could do some changes to all student/teacher users via CSV but accounts had to be manual. So admin was able to do some but not all via CSV.
'SoS', Ken