I had one interesting data point for upgrades for SIN-HEL flight. Originally had upgrade on waitlist from economy to premium economy with points (I wanted to see how the new PE feels) but it was denied at T -37h: Expertflyer showed at that point J4 and W9 so I called gold service phoneline and asked how the situation looks and can I still upgrade to business with vouchers or points. First they said that whole plane is full and there is no upgrade slots at all to business or premium economy but the lady said that she can still put me on waitlist to business even there is no change for the upgrade but I will dont loose anything by trying. Used vouchers for the upgrade tryout and it was cleared at T -5h! I think at the front part there was one J seat free at least during flight.

Will be flying between the HEL-SIN-HEL route still maybe 4 times until next summer due to family reason so this thread for the upgrades was very informative and therefore wanted to sign up and share my experience. All help and info for future upgrades tries are of course helpful!