Thanks for all the comments and advice. Been away from FT for a day and impressed by the gender "war" that broke out in my absence. It was probably not clear from my first post that I was NOT looking for avios upgrade before seeing the offer just before OLCI. I (mistakenly) did not think at the time that the avios upgrade offer was linked to avios availability.

I called BA and was told I could not get my CW seat back without paying loads of money as the fare class I had was no longer available.

I offered Mrs C the possibility of a swap on the plane if she is not upgraded but she refused the gesture (as I would have done in her shoes).

Very pleased to say, she was upgraded, and both Mrs C and I were very pleased.