How do I change the licensed user name name in outlook for mac?

I have several users on a Office Small Business subscription and one of them ran into problems in Outlook that could not be solved. As a result, I moved him to a different machine. The new machine already had Office installed, but was licensed to one of my other users. When I installed  my current user's mailbox, all of his outbound emails carry the licensed name of the previous user even though they show the correct email address and refer to the right exchange mailbox. I have tried uninstalling Office and removing the application reference files in the library and reinstalling using the correct user's Office 365 credentials, but it did not replace the licensed user name and still shows the wrong license name every time it opens and each time an email is sent. Any ideas on how I can solve this? I need this user's name to reflect on the license. Does anyone know if there is a way for MS to clear the license activation on that machine so I can make sure it is tied to the right user?