Another feature.

If you call in and request a domestic upgrade using a SWU, then the agent screws up and marks it as a CR1 request. (I did this because I don't think that I will need to use my SWU by the time they expire) You get to the airport and realize that he CSR made a mistake. You ask the checkin agent to correct it and guess what - they can't. They would have to release your upgrade seat and you get back in the upgrade line. I understand this prevents people from the old using a CR1 and pulling 500 mile trick - but I was going the other way.