So I was told by the Clip Studio twitter people that while you can transfer a Manga Studio licence to a CSP one, you couldn’t do it with a MS EX licence to transfer to a CSP EX one. However, I tried it and it worked!!
So yeah, if you have a Manga Studio EX key, what you do is, you download Clip Studio Paint and then when you open the program, go to Help > Register Licence and punch in your MS EX code. It worked for me and I’m really glad I didn’t have to buy another licence.