It seems that there are only 2 ways to update a user's email address.
1) I have to create a new account, have that user accept the invitation, and then merge those two accounts together
2) Have the user login and update their email address.
While it seems simple to have users go in and update their email address, this is more difficult when you are doing a mass update to update everyone's email to a UPN. It's also not any easier to go in and create 400+ new accounts using the UPN and then ensuring everyone accepts the invite and then merging the accounts.
As an admin, I should have the ability to go in and update the email address either via the API call or by using the User Management.