I am sure I have undone a UuA before and I think what
mikefly has outlined was my experience.
Also see here
http://www.britishairways.com/en-gb/...ditions-of-use
13. The fare conditions for the original ticket purchased will apply, please check Manage my Booking or call your local service centre. In addition to the fare conditions, specific to the Upgrade using Avios points:
1. Changes to the date or time of an Upgrade flight must be made at least 24 hours prior to the scheduled departure of the relevant flight.
2. Changes are subject to availability in the original cabin, in addition to the upgraded cabin. Certain flights may have very limited or no availability on certain dates because of existing bookings. If the original booking is peak and the customer wishes to move to an off-peak date then the difference in Avios will be refunded. Equally if moving from an off peak to a peak date, the additional Avios will be charged. Where there is no availability in the desired flight, Avios points will be re-credited to the Member’s account subject to reward flight booking and service fees applicable which will need to be paid by the Member.
3. Within 24 hours the Avios used for the Upgrade are not refundable, however the customer can be downgraded back to the original cabin should the fare conditions allow and subject to availability on the desired flight.
4. Cancellations at least 24 hours prior to departure of the relevant flight, will be eligible for a full refund of the Avios used for the Upgrade, and the fare conditions of the original booking will apply for any other refunds.
5. Cancellations within 24 hours prior to departure of the relevant flight, will not be eligible for a refund of the Avios used for the Upgrade, and will be subject to the fare conditions of the original booking.