Making a new transformation spell

[Coldshard]

How would I go about this?

I would like to have a spell that turns one creature into another. If possible I'd like the spell to be limited in who can cast it (only specific creatures) or to have a varying result based on who casts it.

Is there a way to do this with a mod? It looks like 'Effect 10044' is for transformation but I dont know how to modify the rest to make it do what I would like to do.