The best thing to do though, is that once your sure you want to change, contact your school ASAP.
I would suggest you go to talk to the person who is in charge of the options/timetabling etc etc -for my school that is our deputy head.
You may have to get a letter from both teachers (from the subject who would have been teaching you and the one you will be getting if the change is 'ok-ed') saying that it is alright for you to make this change.
It may also depend on the size of the class that you will be moving into, as if it is already too big they might not want another student in the class.
My friend has recently decided he also wants to change subjects and has set things in motion. I don't know what's happening with that but we'll soon see.
P.S Make sure you're completely sure you want to change as once you do I daresay they wont be willing to change it again. My sister did music too, and I think it was quite tough but if you're good at it (composing and listening etc etc) then go for it
once again thanks for all your support