I know why the system won't let this happen but it's actually a discussion I had many years ago with a contact who worked there at the time. I can totally understand why other combinations of upgrades shouldn't be allowed because they're in effect cheating the system, but for somebody who has used an RU or an APD upgrade I personally think a OneUp on top should be allowed. It's a hard cash offering and is only going to be accepted if there are seats available after RU's and APD upgrades have been processed. I can't see systems ever being changed to allow this though.