During our recent migration to Exchange Online in Office 365 we had to manually export our old mailbox to a PST file and import to the new mailbox.
This involved creating a new account within the users Outlook profile, doing the export and import, making sure the new mailbox synched to the cloud mailbox, switching our email delivery (MX records) over to Office 365, copying any emails that had arrived in the old mailbox since the PST import and then delete the old mailbox account. Pheww!
As you can imagine that took some time even for the modest 7 users that we had. So it really annoyed me when I discovered that at the last stage Outlook informed me that the old mailbox could not be deleted until all other accounts were deleted because it was the primary account and even more annoyed when I discovered that the Primary account cannot be switched.
But we sorted it out in the end:
Rename the mailbox OST files which can be found under c:usersusername….AppDataLocalMicrosoftOutlook
Delete the new mailbox account from settings (Control Panel, View by Small Icons, Mail).
Create a temporary outlook data file under the data files tab (Outlook will not delete the primary account until you have an alternative data file present).
Delete the old mailbox primary account.
Add the new mailbox again.
Outlook will create a blank OST file – immediately exit Outlook as there is no need to sync up again.
Delete the newly created OST file.
Rename the new mailbox OST file back to the original name.
Your mail will re-appear and the new mailbox will be the only account present and therefore the Primary account in Outlook.
Remember the delete the temporary outlook data file you created otherwise this will continue to appear in your outlook folder lists.
NOTE: you can of course create separate profiles in Outlook and swap between the 2 and this would be the prefered method of achieving a migration if auto synching of mailboxes was available but we needed to view both mailboxes side by side so that any new email arriving during the cut over period could be easily copied to the new mailbox from the old.