Originally Posted by
1k650
I guess Instant Upgrade means only when NF or NC space is available, correct?
Nope. For Y class tickets, "space available" means any first class seat (F>0). For B and M class tickets, "space available" means any first class seat in the PN bucket (PN>0). PN is a subset of P (which is a discount first class fare).
However, both of these usually have much better availability than R/RN class (bucket for CPUs (old NC/NF)) on UA.