I always thought that if you adjusted the "keep at most" number in an SP, that it only affected go-forward recordings. For instance change KAM set to 3, from 25. If you already have 5 episodes, it would leave the 5 and not record again until you are down to 2.
Apparently, though, that's not the behavior? I had an SP that was at "record all" and had 100+ episodes. Wanted to stop/slow the pace of recording additional episodes, so changed the KAM to 3 and it automatically deleted all but three of the unwatched episodes. I guess in hindsight, I should have just switched the SP to "new only" as there are no new episodes, only repeats.