These were my steps to extend my FFC.
In late December I used my FFC (May 2026 Fly by date) to book an international Economy flight costing few dollars more than my available FFC credit with the intention to cancel after 24 hours and hope it would extend the FFC expiration date. When booking I saw that the UA flight calendar had all dates beyond the FFC May 2026 Fly by date grayed out and blocked out.
After reading posts by mitchell, WineCountryUA and Lurker I decided to not cancel, but change my booking instead. Key point being to use the FFC as a form of payment, not as a starting point. Then try the change date past expiration.
For this I opened my exisiting booking and entered a different international destination and to my surprise found the UA flight calendar showed all dates available now, even past the May 2026 Fly by date. After selecting new dates, I used the FFC credits from the exisiting booking to book a new international Economy flight to a different destination.
From the last booking I had some FFC credits left over that now have a new January 2027 expiration which also is a surprise.