Help Excluding Pokemon based on Pokedex ID


I want to make a game where there are multiple regions to explore, but whenever a new region is unlocked I want to create a restriction that you cannot use Pokemon from a previous region until certain conditions are met. This way the game remains challenging and encourages the player to use new Pokemon. I was hoping to do this by indicating that:

If you have a Pokemon say with an ID of 1-151 you cannot have it in your party within that region until you have met a condition. And also because you are entering a new region make it that all Pokemon in your party get transported to the PC. Kinda like starting over with a new starter, except you have other Pokemon, they just can't be taken out of the PC until a certain point or only if you're in the previous region.

Let me know if you have an answer for this or if I should post this somewhere else.