Originally Posted by NickP 1K

PN is needed for B/M Instant upgrades
JN for Y Instant upgrades

Both of these are searchable on expertflyer; normally also from what others reported if you purchase on united.com you DO have a chance to confirm the instant upgrade prior to payment final click through. Through your travel agent or offline ticketed you should use expertflyer.

PN seems to match ZN which is standard First/Biz Awards - This also seem to match regular farecodes A and P

JN seems to match regular seat availability in F - so a Y instant farecode upgrade means if a rev seat is available it's yours (so it seems)