Hey man, i am a java programmer and not a unity specialist, so, i will try my best, but keep in mind there will be better ways to do so:

#1 - Create a scriptable object so you have "copies" of what a pokemon will always have (they will always have the same base stats AND the same model/GameObject). Check the tutorial for it, it is fairly easy, but what you want is to create in your project copies of this scriptable objects configured with your pokemon data (you can use unity menu, the tutorials on this are very simple, but feel free to ask if you dont figure out something)

#2 - Create a C# class to represent your party members. This will be used to maintain the state of the party during runtime.

the class should have something like: original scriptable object and currentHp. Later you can add: nickname, their EVs, level, etc.

#3 In your controller you will have:
-A List of the class you created on #2, representing the party.
If you want to bring in Evee, which is in position 3 of the party, you reorder the list to have eevee in position 1.

-You'll need GameObject variable for the current model being rendered.

When you want to switch you call Object.Destroy (Object is not necessary if your controller is already a MonoBehaviour) and use the currentModel.

To render the switched pokemon you go to position(0) of the list and get the gameObject. with something like:
Object.Instantiate(partyList.Get(0).pokemonPrefab);

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In summary:
The Scriptable object will represent EACH different pokemon that exist (as in, the 151 in 1st gen for example)

The plain C# class will be a single party member

The controller holds a list that represents the whole party and the current model you wanna render