Remove the primary account by editing the registry

You can edit the registry to remove the primary assignment, however it is not supported and not recommended by Microsoft. If you mess up, you will need to make a new profile, restore the profile key you exported, or use System Restore to go back to a previous restore point.

Close Outlook and open the registry editor.

Press Windows key + R to open the Run command then type regedit in the Run field and press Enter.

Tip: Export the profile key before editing, so you can recover your profile if you make a mistake.

In Outlook 2016, the profile key is at:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Profiles\profile-name

In Outlook 2013, the profile key is at:
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Profiles\profile-name

To remove the Outlook 2010 primary account from the registry, go to the profile key:
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\Current Version\Windows Messaging SubSystem\Profile\profile-name

Export the profile key then search for and remove one (or both) of the registry keys related to primary account.

I searched the profile for 001f662b (or 001f6641 in Outlook 2016) as this value is used by each account. You'll find two keys containing this value for each account and you need to delete the second key that belongs to the primary account (you can delete both keys). Once I did this, I could delete the primary account from the profile.

To verify it's the correct account, either look at the alias in 001e660b (the alias is at the end of the data: /o=ExchangeLabs/​ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/​cn=Recipients/​cn=74479d8714d3414c8502650cc962e1c6-maryc) or double click on other keys and look for the address. For example, in the screenshot below, the address is in 001f6641.