I've been on the phone with microsoft for over an hour trying to get a straight answer from them, and if I was set up to test this myself right now I could find out.
I'm deploying o365 SSO in stages by OU, dir sync is performed every 3 hours. I need to do a sync right away.
Will 'start-onlinecoexistencesync' only sync those users that have already been federated, or will it force a federation of all users?
finally got an answer out of them... Yes, it will only sync those users that have been federated