Originally Posted by sfo789

If sked flight is delayed, you can call in / go to a club and ask them to confirm you on earlier (doesn't have to be P). At that point, they should endorse your ticket as INVOL, meaning that you can call in to UA and request Original Routing Credit for the segments you "should have" flown in the class of service you should have been in.

Plus, if there's F but not P upfront, an agent can force you into the front cabin and if you're lucky you might get 3x PQM