I have a companion pass that expires at the end of February, but will have a hard time getting a trip in before then. It's my understanding I can book a trip with the companion pass and cancel it, then it would be under a future travel voucher (FTV) that would not expire until April 2023. When I go to rebook the companion pass trip using the FTV is it possible to change the destination and class of service?

For example if I book ORD-LHR in J for this "dummy" booking, cancel it, get the FTV, then 6 months from now I am ready to travel using the FTV companion pass will it let me book something like ORD-LHR-JND in F while of course deducting more avios out of my account?