You can cancel any avios booking up to 24 hours before the flight; if the amount of cash you paid is less than the cancellation fee you just forfeit the cash you paid. You can keep checking to see if CE reward availability opens up, if it does just call in to BAEC and they can collect the additional avios and rebook you in CE. If reward availability does not open up you can hope for proactive online upgrade (POUG) as noted above, or ask if there are any airport upgrades (AUP) when checking in on the day of flight. POUGs and AUPs are upgrades with cash.
https://www.flyertalk.com/forum/brit...24-thread.html
https://www.flyertalk.com/forum/brit...24-thread.html