Book Flights, Hotels, Holidays, Car Rental with British Airways - BA.com exec club log in
manage my booking
my booking
change date/time of my flight or
cancel/refund my flight as appropriate.
you should be able to change to another club class seat on another flight if there are any available.
if you have to move down to the cabin you paid for you are not using any miles to upgrade that sector so you should get them back unless you are within the minimum notice of change period in which case you won't.