I think it is a bug. My understanding is that BA.com shouldn't show upgrade option on already upgraded tickets. They may have changed their practice, but my suspicion would be a bug. If you are keen to fly First (and I agree not necessary but it is very nice) then they might have an offer after check in at the special upgrade desk (nearish the lifts to the train to T5B and C.) This would be a fixed price cash offer (£399, £499 or something). AIUI you would be allowed to do that as it is essentially a BP swop.