Originally Posted by WineCountryUA

Because UA has a policy allowing the agent to refare to the lower fare in the first 30 days for a $50 service fee

Not that simple. You can not use the FFC to rebook the same flight. So you need to cancel, book a different flight for the same or lower fee, than cancel again and rebook the original flight for the lower fee. Along the way in one or both rebookings you will pick up the credit.

You can ask why UA makes this difficult BECAUSE it is not in their interest to let you do this.