The easiest answer is to use a four-year rolling average of teams from the MaxPreps rankings (power points). That way, if a team in 3A has one excellent year, but three below-average years, they might move up, but they probably won't. If that same team is in the regional finals every year, despite not winning a state title, that consistency means they are ready to move up in my opinion.
You are moving about 10% up and down, which means you have to be elite in your classification or at the bottom of your classification. Classes would become smaller at the top of the pyramid and larger at the bottom. Thus, to play with the elite teams, it is hard to get there. 32-32-64-64-128-128. The 128 team classifications would allow many of these SSIAA schools to face similar type programs. If a team is "recruiting" then their success will force them to move up and away from other teams who don't do that.
So I've pondered on the idea of a promotion/relegation system for a while. It has its merits. However, what criteria is going to get used to determine who moves up and who moves down? Many of us go through cycles where we're up for a couple of years then back down. If you don't follow a particular program, it's hard to know what they're going to have from one season to the next. Also the option of going independent or playing a couple of easier games is something you can do now. Never have I, or anyone I speak to, says because we or another team had a bad year they should drop a class. Neither should we move up a class just because we made a playoff run.