Last few months I'm responsible for the migration of Office 365 to Office 365. While doing so, we came into a situation where users have their old mailbox as Primary Account and new Mailbox as their secondary account. This is a quite common scenario that people are running into and something that is expected. Usually, my recommendation is: Please create a new profile for user and topic is closed. It's also quite easy to achieve this in an automated way where you delete all profiles and Outlook just goes with autodiscovery adding new account as required. That's how I have always done this till now. My Client has gone thru setting up 1000+ users with their second account in Outlook and deleting a whole profile, recreating would cause lots of downloading of emails from Office 365 that my Client wanted to avoid.