Here is the text from the poug. "Promotional upgrades are non-refundable and the original fare conditions apply to any upgraded ticket."
And "Customers who change their travel arrangements after the date of booking will not be eligible for this promotion unless their revised travel arrangements also constitute a qualifying flight. In the event that the flight is not a qualifying flight, customers will travel in the cabin they originally booked and if they wish to travel in the upgraded class they will be required to pay the normal upgrade fare applicable in accordance with British Airways standard fare rules. The amount paid for the upgrade can be used as credit towards the new upgrade fare."
so appears change based on you original ticket. Plus the change appears to be to a qualifying flight in your case