Ok, that helps. Is this the return portion of your ticket or is it the outward? If it's the outward then the whole itinerary will be repriced based on current fares and you will almost certainly pay a change fee as well. The easiest way to ascertain this would be to do a dummy booking for the itinerary and you'll see what the current fares are. If you have already flown the outward and this is the return then any changes should be calculated based on the historic fares at the time you made the original booking. Almost certainly there will still be a change fee. The exact amount will depend on the precise fare rules that apply to your ticket.

I am not sure why BA are saying you need to change to a fully flexible ticket? They should be able to reprice the original booking based on the date change and tell you what that is, and then apply the upgrade. It may - possibly - work out cheaper to simply upgrade using cash.