you entered into a contract with CX to fly you somewhere not to give you points, i can't see how you can prove any loss there

havent't gone through the T&Cs of the BAC (which would have probably been updated a few times since you signed up) but I'm pretty sure they have all sorts of caveats around BA being able to amend/remove any benefits at any point, I'd be amazed if this had any legal standing whatsoever

that said if customer support incorrectly told you you'd get the old points, raise a complaint, quite possibly they might honour it as a goodwill gesture