Due to fly AMS-GRU tomorrow on KL - I see one seat in C, but no upgrade offered during OLCI. I called KL to enquire - they could book me into that seat by changing my ticket for a mere 2500 EUR. I'll check at the airport again tomorrow.

Where do you see this one seat? it is very likely they could overbook you in J for 2500€ and that there aren't actually any seats available if everyone shows up.

I see the seat in Expertflyer, and they have confirmed the seat is available.

Actually I see J2 C2 for that flight, but it just means there are available tickets to buy, it doesn't say how many tickets were already sold.

I don't know where you're looking, but EF shows J1 and everything else zeroed out on KL791 tomorrow.

hrm it is indeed J1 and 0 for everything else now, so even better, unlikely that there is an actual seat available, it just means they are happy to overbook it by 1 more seat.

Weirdly though, if I try to book business class for that flight on KLM.com now, I'm quoted 6000-odd Euros with a note "only 2 seats available at that price", but no option to select a seat, which corroborates the idea that it's actually full.

You can't select a seat because the flight is already "in the checkin system", EF doesn't have NL POS, so that could explain why you see 2 and EF shows 1 (or it could just be "fluctuating")

I checked again about half an hour ago, and all of a sudden 6 seats had opened up in business class and I managed to score one for 454 EUR.

OK, not a bad price. What was your original fare class? And was there a mileage option as well?

There was no mileage option that I saw. The original fare class was K.

It's now T-50 or so hours for my SIN-CDG flight and the following message appears:

Checking award availability, classic awards are still available in W and C which is of course no indication for upgrade availability. Well, next stop is T-30 for OLCI, will see what's on offer then.

SIN-CDG for October 2nd currently have following availability:
POS France: P4 F3 J4 C4 D4 I4 Z4 O1 W7 S7 A7 Y9 B9 M9 U9 K9 H9 L1 Q4 T9 E0 N9 R8 V7 X6 G0
POS Singapore: P4 F3 J4 C4 D4 I4 Z4 O2 W7 S7 A7 Y9 B9 M9 U9 K9 H9 L1 Q4 T9 E0 N9 R8 V0 X0 G0

So it seems quite surprising that there aren't any seats available for upgrade

Another update (TXL-CDG) : between H-30 and H-4, I looked regularly at the seat map and sometimes the C upgrade was proposed and sometimes not, and when proposed it was always €119 with 3 seats available. From about H-4 to flight time, the upgrade was not proposed anymore and all Y seats were taken on the seat map. Finally, I got a free op-up at the gate ^ :D

Yes it is indeed weird. I‘ve sent a DM via Social Media to AF, wondering what their reply will be. An interesting thing: before T-56 many seats in Y were available, but at the time I logged in, a lot have been taken. Currently I have 33H with G empty, E & F taken, Fingers crossed that either mysteriously an upgrade will be available or at least 33G remains empty 😃.