How to bulk change a user's logon domain (UPN)?

We have a non-routable domain as our AD and to tie into Office 365 we need to add the UPN suffix for our publicly owned domain. I can do this manually in AD, user properties, account tab, and then changing the drop down to the other domain. I tried this in AD Manager plus and cannot get the change to take. I exported a list of AD users to CSV, edited the list to have sAMAccountName and userPrincipalName columns for the users I need to edit. The userPrincipalName column has the new/proper UPN set. So I tried to do a bulk modify using the CSV to import the list, which shows up fine. But when I do the edit it blanks out the user logon name and UPN. Is there a way to do this bulk edit and change the UPN for a list of users?