How do I make a code that can transform the player into a different mob

I don't think this is possible. 

The code is possible, but you would need to learn Java first, then write all of the elements for this feature yourself.

I created a test mod to transform you into pokemons, I explain how I did it, first grab the java model made in blockbech (I clarify, make a folder where you put all the cubes of the model so that it can work) then export it, then I put in mcreator to make an armor and then I put it as boots and in the [CLICK TO EXPAND] section next to that you have to choose the folder where you put ALL the cubes, where is the L or R section, you can put the legs of the model or you can put the same folder where all the cubes are depends on how the model is, in the texture part you are going to put the texture of the entity model so that it has texture, at the end you edit everything as a normal armor but in the section from [triggers] put in (part of the model's armor) tick event, you create a procedure with the invisibility potion, I hope I have helped you = D