For anybody who is interested in the outcome, it is exactly as many suggested, just the fare difference plus change fee.

I've been waiting because the fare kept changing a lot and I've been looking every couple of days, it was pretty high and I almost decided to knock it on the head, get the remains of a refund and pick a new itinerary with BA, however BA's price doubled and Cathay's came down to a fare change that was manageable.

So my advice is to others is to hold on and wait until you can because CX prices seem to change a lot more than some competition just on this routing for example.