How do I change an office 365 email address and avoid data loss/re-syncing in outlook app/desktop

[Jake_Mitchell]

Let's say we have a registered user account who's current email address is user@whatever.com. There is an alias on the account called events@whatever.com. Now, the user wants to use events@whatever.com as their primary account for sending/receiving emails (because their assistant also sends emails using the account on another device) and to have user@whatever.com as the alias.

At this point, the user has both Outlook on the desktop and as an app on their cellphone. I don't want to incur massive data transfer costs after converting the email address on the user's account, so how do I do this switch efficiently? 

[#2]

Are those both added as proxy addresses on the *same* mailbox? If so, all you need to do is change the primary SMTP address, nothing will change client side. If you decide to also change the UserPrincipalName, this will change the way the user logs in, so make sure to inform him accordingly. But no data will need to be resynced.

[#3]

Currently user@whatever.com is the main mailbox (and user account). events@whatever.com is the alias. I want to reverse that scenario.