[Uh-Nu-Buh]

I believe you can change magic sites by Map Modding. This is a whole different thing. I haven't done much of it myself, but Gandalf would be a good resource--or you could repost your question with a different subject heading to get those interested in Map Modding to take a look.

I may be wrong. It could be that you can just change which starting sites you get. I hope you can mod the sites themselves though!