How to Change UPN in O365 for a Mailbox User

There is only one way you can do it while running the exchange and O365 hybrid mode. this is not directly possible but you need to first change this to intermediate UPN and then to the required UPN as below 2 power shell commands:

Let say you want to change UPN for a user user@domain.net to user@domain.com,

Set-MsolUserPrincipalName -UserPrincipalName user1@domain.net -NewUserPrincipalName user1@domain.onmicrosoft.com
Set-MsolUserPrincipalName -UserPrincipalName user1@domain.onmicrosoft.com -NewUserPrincipalName user1@domain.com