I usually play on epic, so when I participate in a challenge, I come across the same issue. What's more, going from long to epic and back to long has the potential to change the number of days a sim has left in a stage. If it were 30 on long, then you play a neighborhood with a different setting, then go back to your long lifespan challenge family without changing to long first, you may or may not have 30 days remaining for that sim. At least, that's my experience. Of course it can affect the epic sims' lifespans, too.
My solution was to create a test neighborhood. I'd actually downloaded a custom 'hood that would load super fast (because it was small), but you can use any of them. I don't play there, except to mess around when I want to try something out. When I move from one neighborhood to another, or if I'm loading the game and I'm not sure where I left off, I go to my test 'hood first and change or confirm the lifespan setting there. Then, when I get to the neighborhood that's really important, the setting is as it should be with minimal chance for those accidental changes.