AD Connected Office365 default domain change

tsears 

With AD connect in place, on premises AD is the source of authority for the accounts, so you will need to change the User Principal Name in on premises AD, and also change the primary SMTP address in either on-premises Exchange if you have it, or AD Users and Computers, enable Advanced Features, and modify the uppercase SMTP: entry to the new domain address, and if you still need it, have the original email address here as a lowercase smtp: entry which will keep it as a secondary email address,

AD sync will auto sync every 30 minutes, or you can trigger it via PowerShell by running the following command on your AD Connect server;

Start-AdSyncSyncCycle -PolicyType Delta

I'd suggest testing this with a couple of accounts first just to test out it's going to work OK for you.

You should not need to create a new outlook profile.