I did some copy and paste here and there in your mod and make possible to subclasses random appear after you terraform a planet into a different main class.

For example, after terraforming a planet into the desert main class, this terraforming can result in any of the desert subclasses (including the original one). The change of each subclass be choosen is the same we have in the galaxy creation.
It is not possible to choose each class you want. I don't look way way to do that because I like the random choice.

If you are interested, I can send you the modified files, so you don't have to redo what it has already been done.

There is only one small graphic bug I found, when terraforming results in a new subclass the animation the animation on the planet thatused to accur after the terraforming doesn't show. It is like the set entite command cut this animation for some reason.