I am speaking from direct personal experience. I had a LAX-ORD-CMH P fare last month, and the LAX-ORD leg was cancelled for maintenance. The SHARES auto-rebook engine put us on a red-eye LAX-EWR-CMH itinerary in coach.

I called in to the 1K # and it took over an hour to find an alternative itinerary with F seats LAX-ORD-CMH for the following day. Hence my original comment about talking oneself back into the front cabin.

I had to persevere to find/persuade an agent to dig into the reservation to find the -UP/-UPDI fare, find F space on a different itinerary, and get supervisor authority to reissue the ticket for front cabin space. What I do know for certain is that the auto rebook engine puts P fares with -UP/-UPDI underlying fares into coach during IRROPS.