World Phases

Hey yall,

I am currently puzzling over the problem of creating world phases via certain milestones being achieved.

I could, for all purposes, simply check for existing achievements and set a variable to that, which then controls what the game does, but I don't think a script checking so often would be conducive to the game running well.

That is why I am asking if there is a way to permanently store some kind of variable or hallmark, to control things like preexisting mob behavior, textures, and mob spawns. 

Much appreciated if controlling preexisting mobs would be possible and changing their textures.