Yes, how to refuse an operational upgrade in Club Europe.

I saw something during my last boarding with BA that made me think.

Starting point
As a rule I fly in Economy inside Europe (and I fly 99,99 % of the time inside Europe, i.e. 320 or 737)... so I try to pick up good seats via the OLCI. An example, the first row in economy A-C because of a 2+3 or the F seat in the second emergency row and for sure trying to avoid as hell E or B seats.

My question
The person just behind me in the line was stopped at the boarding gate and informed she got an upgrade to business due to a very full flight. I missed the end of the story... I was walking away but first I got envious but later I considered that a business E seat is not something one could really appreciate. If it will one day happen to me how can I kindly refuse and still go on with my nice economy seat. I have the feeling her previous seat was already 'gone', so how she could have managed a denial ? Do you have any experience and any first hand experience to share ?

Thanks a lot.