Some reports from my recent RPU adventures:

1. Applied RPUs for 2 passengers on a Transcon (west coast, to IAD) + short-hop (100 mile flight), the short-hop cleared 1 month out, the transcon never cleared. Both RPUs returned automatically 1 day after the flight. (I don't know if I would've cleared the CPU on the second flight, the upgrade list was 20 people for a CRJ-700, I was on a lowish fare).

2. Applied RPU to a similar transcon + medium hop (500 miles), both segments cleared a few weeks out. Had irrops on the transcon, decided to cancel the second segment. Cancelled online, my RPU was returned (the website told me I'd get my RPU back during cancellation, even though I'd flown the long segment).

It's unlikely that I'll be able to use many of these upgrades over the next year, so there's no chance of my being able to use that extra RPU I got from (2) anyway even if I wanted to (but I'd consider that unethical regardless).