The 5 day complimentary FC upgrades are on a space available basis with the space having to be available in R class. The Y-UP fare or the guaranteed UG at time of booking for an elite buying Y class can come out of any class. So yes, there could be two or 4 P or F seats available and 0 R seats so someone could purchase a coach ticket at Y and be assigned a P or an F seat while you wait for an R to become available.