If you still use the old email address, try this:
Open the following link:
https://appleid.apple.com
and select "Manage Apple ID". Login with the old ID and password. After logging in, select "Name, ID and Email Addresses" (1). Go to "Apple ID and Primary Email Address" (2) and tap "Edit" (3):
Now enter the new email address and save it:
You'll need to verify your new address.
When you open iTunes (computer), you'll find the Apple ID with the old address in iTunes Store somewhere at the top. Tap it and sign out, then sign in with the new one.
Hope that helps.