Can't Open Outlook on Web after changing Primary Username in Admin Center

luckman212 The short answer is, as you have probably discovered, you need to wait up to 24 hours after making changes to email\accounts to have Outlook\outlook web work correctly. 

Long answer- I am currently preforming a tenant-to-tenant email migration at the moment.  The domain is being migrated as well.  The new\destination accounts are currently using email address removed for privacy reasons domain.  When the domain that is being migrated is removed from the current tenant and added to our tenant the accounts will need to be changed to the migrated domain.  I created a test account in my tenant.  The tenant already has multiple domains.  I logged in as the user in O365 and opened outlook, word, etc.  I then changed the primary email to a different domain on the test account.  This changed the primary email and the username.  It also set the first domain as an email alias.  I was immediately able to sign in using the new email\username.  All apps opened fine except for Outlook.  I had to click the username 10 times, then refresh the page and it finally opened.  This occurred for 9 hours. After that whatever needed to sync, replicate, or whatever had done so.  I saw similar articles online stating that it can take up to 24 hours for "Exchange" changes to replicate even in O365.  Always test things before the actual changes.  We know what the changes should do but are never certain of what they actually do unless we test first.