This has been a very interesting read. I must say however that even if there are guidelines. There are way more scenarios that don't always fall within set guidelines. It is never nice to have to downgrade anyone. If the aircraft swap happens long enough prior to the flight, res tries to notify passengers if time permits. As far as who gets downgrade. I am under the impression it is Prestige/Elite and the Super Elite. That being said, I believe that sometimes res is instructed by departure control not to move anyone if they are already checked in. OF course what happens by the time everyone shows up at the airport is a whole new story.
One thing for sure. No matter how it gets done. There will always be some that won't agree. The previous posts should prove that.