An update on my plight for a change to a P.S. flight in premium cabin. Checked the app every half hour and at T-10, it had a routing with a LAX-EWR segment which I was successfully able to change with only the $75 SDC fee but it wasn't the segment I wanted. I confirmed it anyway just to be sure I got a P.S. seat. Stopped by the LAX United Club to see if they could change it over to the one I wanted and was successful in confirming that without being charged another $75

So a sort of DP here, the app and system will sometimes let a P.S. routing segment slip through the cracks and can be confirmed 100% with only the fee to be paid, but YMWV. Just keep on looking at the SDC options every hour.