Affecting spawned animals retroactively is a bit difficult unfortunately... some people might not be happy if they lose their animals just because they changed a slider in the settings
Probably it's better to use the deletenpcs console command instead: first change the slider, then use the command to delete all npcs (maybe reload the world then to trigger a respawn of new animals)
that's the one!
actually if you start another world the you got the spawnrate that you chose, so don't need to deletenpc