How do I change my old iCloud email address to a new one without disturbing or losing my iTunes account?

I opened an iTunes account a few years ago when the program 1st became available. Problem is I used a email address from a company that is no longer in existence, however it was once upon a time. My iTunes still recognizes the old address, but I can't retrieve emails because I've tried to log in with no results. I would like to change my iCloud account to my current email address so I can back up my info, but without a way to veiw old email it is proving impossible. I would also like to change it without losing or disrupting my iTunes account and info. Also, I'm having to do this by iphone, cannot get access to a desktop or lap top at this time. Any help or suggestions would be greatly appreciated.