It is starting to sound like BA may be promoting some sort of fixed price special offer for the sector upgrade? That would probably be a bolt on to an existing BA ticket for that sector.

But QR will be having to reprice and reissue the whole ticket. If you haven’t flown the outbound yet that would mean repricing the full return itinerary at today's prices, with either the full connecting journey on the way back in the higher fare bucket (journeys are priced between the origin and destination, not per sector), or as some sort of multicity itinerary.

So that’s not comparing like with like in terms of ticketing and pricing.

If you want the BA offer you'll have to rebook with them, but the total cost of cancellation penalty plus upgrade plus any fare difference vs your QR ticket may make it not worth it.