I have a flight in a couple of weeks and 1 RPU and I'm trying to decide if I should apply it on the IAH-SFO-PDX or PDX-IAH (red-eye) leg. The outbound is on a Sat and the return is on a Sunday night and right now R=3 on the IAH-SFO (752), R=9 on SFO-PDX (738) and R=8 on the PDX-IAH (320). I guess the real question is as a plat on an L fare, how likely am I to clear the Sunday night redeye on a CPU?

If I applied the RPU to the return leg and the SDC'd to a different return flight, would I just end up on the CPU waitlist for the new flight or would I have any priority from the RPU being applied to the old flight?