i did this for a nyc-lhr flight in december. i booked lowest economy over the phone initially so didn't get a POUG online. called the executive club (bronze) and was able to upgrade the return to WTP for around £130 and then 10k avios to CW. they waved the telephone and ticket change cost of £100.
I'd suggest giving the BAEC a call - they should be able to help and advise on the availability of the avios upgrade to CW. they've been extremely helpful whenever I've rung about this.