Usually it's very hard to get out of a downgrade situation, but the silver lining is that you still get to fly premium economy and you can claim UK261 and get 75% of the tickets reimbursed once you land in London (obviously if the ticket price includes London-Zurich, you need to multiply by the ratio of distance but since PDX-LHR is a much bigger distance than LHR-ZRH it should be very close to that, and claim once in Zurich). I would not rebook since BA will have no legal obligation (well, it's a blurry zone) to reimburse your flight unless you had fully cancellable tickets. Take the downgrade with the money.