This is likely a very basic question so I'd appreciate your patience and any help. I bought 4 premium economy seats for my family to travel from SIN to SFO in Feb. 2020. The outbound flight from SFO to SIN is J class on United metal thanks to a friend's frequent flier miles. Given the VERY long flight, I am thinking now I want to upgrade our seats to J class on the return flight. We loved our J class experience on SQ to NRT last year (bought with a combination of Chase points cash on the Chase travel portal) and I'm worried we will be miserable on the 14.5 hour inbound flight. We just did an Air France flight in Premium Economy and none of us slept at all on the 10 hour flight.

We have no status on any airline and only fly long haul 1-2/year at most. We have Chase Sapphire Reserve points that I could transfer in to KF, but I don't want to be stuck with nearly 300K KF miles if it's unlikely our upgrade request is granted. I bought the ticket on Kayak through a third party, and neither the e-ticket or receipt (from Kayama) show the fare class so I'm assuming it could be up to the 73,000 miles/person to upgrade (Advantage rate for booking classes S and T). Are 4 upgrades from PE to J highly unlikely? Is there some other way to figure out what our fare class is?

Premium Economy is nearly sold out, but I was able to see that I could get 4 J class seats for cash ($8K each for a one way!). Yet I'm not ready to pay that much cash. Does SQ ever offer more reasonable cash upgrades to J class?

Any advice would be greatly appreciated!