W
You can switch over the Primary email associated with the Microsoft Account... I just went through this as I am going Google Free.
On a PC Browser, log in to your Microsoft account. Add a new email. Make it primary. wait about 24 hours then delete the old address. It then takes about 48 hours for the servers to all catch up and the change will be permanent on your phone as well as everywhere else. You do not loose data or purchase history.