Look and keep looking. Just in case you don't know, keep trying dummy F redemptions to check for availability. If they've just changed the plane type, there may be one available immediately if you're lucky. If not, check from time to time, especially close to the date of departure.

You won't be able to do the upgrade online. But as soon as you see there's an F redemption available on your flight, you can call You First and they'll sort it out for you very nicely. You could also call the Silver line, but I think You First are a bit better at sorting out on the spot if there is any change in the taxes and fees rather than referring it to the black hole of the Fares Department.

Out of interest what was the aircraft type and route? And how far in advance of the flight did they make the change?