Hi Greg,
I did some tests. It turns out that I can modify my email address into:
I’d like to provide some steps below for your references:
1. Go to Office 365 Admin Center.
2. Click Outlook in the global navigation to access OWA.
3. Logon to Exchange Admin Center. Change the URL of OWA from https://podxxxx.outlook.com/owa/?realm=xxxx.onmicrosoft.com#modurl=0 to https://podxxxx.outlook.com/ecp, and you will go into Exchange Online Admin Center.
4. Recipients> mailboxes, double click one of the display names, in the window of User Mailbox, click email address.
5. Double click the EMAIL ADDRESS, then you can modify it.
If you have any questions, please feel free to contact me.
Thanks,
April Zhou