Originally Posted by
Clack
I'm 6'3" with a sometimes-dodgy back - it's all about the lie-flat seat. Sitting upright for 10-16 hours absolutely kills me. I don't drink, you don't have to feed me much, you certainly don't have to fuss over me with hot towels and other "service." Just give me the flat seat.
If it was all about the flat seat then why didn't you purchase a Business Class ticket to begin with?
As others have advised above it isn't possible to burn Avios on CX upgrades. You're only ability is to earn Avios via the OneWorld partner status BA holds with CX.
Perhaps pose the question in the CX forum and see whether there is the ability to AUP or bid upgrade? Keep in mind that these won't necessarily earn any additional avios by upgrading in this way.