I have done this a few weeks ago from J to F on CX using BA miles. As long as it is the same flight, BA does not charge for change of class either upgrade or downgrade.
So if there isn't a charge, are miles redeposited in class of a downgrade? I have a CX F flight booked but would ideally like to change to an earlier date when no F seats are available. I wouldn't want to lose 25,000 miles, though. Any knowledgeable input would be most appreciated.
update--This may be a theoretical question, as earlier C seats do not seem to be available.