Please see this posting in the thread:
https://moodle.org/mod/forum/discuss.php?d=352457#p1440405
The mass conversion of users from old plugin to core plugin does involve manipulating the DB directly (something not normally done), but one doesn't have to do one at a time.
Before you begin a test ... make sure you have a sql dump of your database ... mdl_user table for sure.
And suggest using mysql to change just one user as a test first. That would be a user whose Google credentials are known so you might have to acquire another google account for testing purposes.
'spirit of sharing', Ken