I have in the past upgraded from PE to Business on AF, on flights to CDG out of MSP, ATL, and IAD. The most recent time I did it was in 2022, flying IAD-CDG. In my experience, sometimes the gate agents announce the availability of paid upgrades, sometimes you have to ask at the gate or in the lounge. IIRC, the only time I could buy the upgrade in a lounge was in an AF lounge at Dulles; in MSP the DL lounge agents announced the upgrade availability, but I had to go to the gate to do it. When I first paid for these upgrades the cost from PE to Business was around $350. By 2022 the upgrade price had gone up to $550, still a good deal in my opinion.