Setting the primary email address for Office 365 accounts is easy to do with PowerShell.
It is done with the Set-Mailbox cmdlet. The primary address is set by using “SMTP” in uppercase in the email address.
You do have to be slightly careful as using the Set-Mailbox cmdlet to change the primary address will remove all of the other aliases. So they must be included with the command if you have any.
I have made this process really easy for you (and myself). Enter the details of the mailbox you want to change below and it will generate the required Set-Mailbox command for you to copy and paste into a PowerShell session.
I have also included all the required PowerShell to connect to Office 365 and disconnect again.
Set the Primary Email Address on Office 365 using Powershell