We were in the family stand upto the point the expansion took place, and we moved to EL3 to be close to my mate. The club sent letters at the time saying you had to move if you didn’t meet the family stand criteria, which we didn’t by then, but I know a lot of people didn’t move, and are still in there.
That is what I would have expected to happen. My understanding would be that you benefit from cheaper S/C's together with your kids in a safe area away from the more boisterous elements but as they reach adulthood, you make way for the next generation of parents with kids of a match-going age. That would mean the S/C holders would know when they sign up that there is a finite amount of time there before they have to be relocated elsewhere in the stadium.