I assume that this was a CX issued ticket and not part of a longer itinerary to Europe.

Your ticket was downgraded to Y and you would have been entitled to some (modest) refund.
Instead of the refund, CX gave you a complimentary opup and you accepted it.
It is hard to see how you could also claim from CX to adjust the information that they transfer to BAEC to reflect that your original ticket was in PE and that you are owed PE TPs.
Basically you got lounge access, flew in Business, but lost 20TP. Not a raw deal.
Or you could have asked for the refund and flew Y. would still have lost the TPs.
If you are persistent enough, you might get a retrocredit for the original ticket. But for 20 TPs?