Apple ID doesnt exisit and cant get pre ordered song

I have changed my Apple ID when i got my phone but on my itunes its still using my old ID which no loinger exists, so my pre order email went to it and now i cant get the link to download the song,how do i get the song and ensure that my apple id changes and stay on my new one! when i do try to download the song itunes says there is an error 5002 and to try again later