You did not, in fact, spend an enormous amount of money for said seats. This is the unfortunate downside to using comparatively inexpensive miles as opposed to cash for premium class airline seats; your recourse is essentially throwing yourself on the mercy of an airline if an airline does an equipment swap (which airlines are wont to do, especially when you plan awards months and months out), as opposed to getting a full cash refund or switching to a flight with seats available for purchase (as opposed to award inventory). This is exacerbated because you aren't even flying the airline who issued tickets (BA), you now need CX AND BA's tender mercies. The rule is that it's the issuing carrier who is responsible; it's not CX's responsibility to do anything, they are free to change their equipment as they need to, as it's BA's problem to handle your downgrade. I seriously doubt CX would open additional award inventory that is not already present, unless you have shiny emerald elite cards in oneworld, like BA Gold or CX Diamond- it might be worth a try anyways, but don't get your hopes up. There's also a chance F inventory opens up closer in, if you are willing to be patient.

FWIW, BA's miles are quite usable on American and Alaska in the USA, and if you are willing to fly coach, offer some excellent values due to BA's distance-based chart if you are based somewhere with good American/Alaska service. So in fact you are going to retain something of value if all you get are some BA miles refunded; 70,000 miles could easily fly you and someone else round-trip to Hawaii on Alaska from the Bay Area in coach (25,000 miles per person), with 20,000 miles left over (enough for some additional Alaska/American flights). So I wouldn't despair that BA miles are useless if you won't fly BA anytime soon.

Good luck and enjoy your trip.