Basically, having a "reseat" every "x" number of years, and in between reseats you can keep your seat - as long as you retain your CC level. If you drop your level, then you lose your seats (though are still able to purchase seats basked off of availability during your priority window). Honestly, I think this is a pretty fair way to do it - as long as the interval between reseating is both long enough to allow people to not gripe about constant reseating but short enough that it doesn't disincentivize people from increasing their donation levels.