I am sorry I missed your first response.   
1. ~2100 users
2. No. Is there a way to pull the old UPN from 365 and set it to the new one?  
Open to suggestions but maybe something like:   
If 365 UPN does not equal ONPREM upn run "set-MsolUserPrincipalName"  
I'm afraid i'm not good enough to create it on my own.