"Whether you pay a service fee depends on what your status was at the time you dropped off your bags. If at the time you checked in your bags you were in the United Economy cabin for seating, then you would be charged a service fee for checking a bag. If you change from United Economy to premium cabin status before you drop off your bags, then you do not pay the service fee. There is no refund for a service fee paid if you check your bags in while still holding the status of United Economy and then later upgrading to United First."

https://www.united.com/CMS/en-US/tra...ggage.aspx#q57