Recently flew MIA-CDG (first AF Long Haul) was booked in PE and at check in when looking at the seat map it gave me the option to upgrade to business by choosing a business seat and paying the seat "assignment" cost. It was €280 for that leg. I was surprised by this option as I am used to having an offer vs just choosing a seat.

I have seen this replicated on my short haul AF/KLM flights as well.

I know it does not really answer your question, but might be an easy way to upgrade at checkin.