I'm not totally sure moving from value to regular is possible post-booking. But 'Priority' itself certainly tends to become costly if adding it at a later stage. It usually starts at EUR 8 and then goes up to EUR 20 or EUR 22 closer to the departure date. It's also possible that Priority is sold out close to the departure date.
Of course, I agree with you there is a trade-off. If you end up not using the ticket, you've had unnecessary expenses on ancillaries. (You might want to read
this thread which has some relevant info on what happens to ancillaries in case of a rebooking under covid rules.)