I would imagine that they would either ask you to swap to a different flight or cancel without penalty.

I am actually booked on SQ33 in Y from SFO in December and it looks like they zeroed out the economy inventory. The seat map hasn’t changed, but this is normally operated by the ULR variant. My guess is they are preparing to swap the aircraft if they are zeroing out both Y and PE. I haven’t received word yet, so I will report back once I hear more. I checked with an agent to see if they knew anything but all they said was that Y is now fully booked. I wonder when they’ll notify me since I was planning on buying another ticket for more passengers but now that’s on hold...