This involves three different *A carriers, so I'll try my luck here before hitting the specific forums.

I'm booking a Eurobonus (SK) award trip on rather short notice, and on the days I want to fly there's no C availability, so I'm flying in Y on SQ (international) and NH (domestic & international). Since this is an award flight the fare class will be X.

So the question is, what, if any, are my options for upgrading individual legs?
According to How to upgrade on *A carriers, both SQ and NH block some fare classes from upgrading (in NH's case only if flying from the US?), but X is not listed. I'm willing to pay cash and take my chances with standby upgrades, although said willingness naturally depends on how much they're going to charge me...

Upgrading from an award? Extremely low chance but it has been known to happen. On SQ I think you are probably more likely to win the lottery.

On NH, not possible to upgrade an award reservation (X), neither with miles, nor with cash. Op-ups I think are rare (happened to me only once in 8 years) and next to impossible if you have no status on NH.

No no, I'm not hoping for an op-up, I'm asking if I can upgrade an X ticket with cash or KF miles.

tide, thanks for the NH info. Does this apply also to domestic routes, ie. I can't upgrade from economy to Super Seat even by paying?

I only fly NH international. You might want to take your question to the ANA forum? Good luck