AD Connected Office365 default domain change

[tsears]

Hello,

I have a client that is currently setup with Office365 that is AD Synced to their local domain controller. Everything is currently working fine but they are going to be rebranding at the end of the month to a new domain name. I have the new domain name setup and is working correctly in their O365, I can change the proxyaddress on their AD accounts to change to the new domain name in O365.

My question is when I want to do a mass change of everyone's email addresses to the new one, can I just make that change in O365 or do I have to change everyone's proxyaddresses in AD also?

Side question: Will they need to re-create their outlook profiles to display their new domain above their inbox or will it auto change that?

Thanks

[#2]

@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.