If the old ID ("email") is yours, and if your current ID was created by editing the details of this old ID (rather than being an entirely new ID), go to https://appleid.apple.com, click Manage my Apple ID and sign in with your current iCloud ID. Click edit next to the primary email account, change it back to your old email address and save the change. Then edit the name of the account to change it back to your old email address. You can now use your current password to turn off Find My iDevice, even though it prompts you for the password for your old account ID. You should then be able to delete the existing account. After doing so, you can go back to https://appleid.apple.com and change your primary email address and iCloud ID name back to the way it was. Now you can go to Settings>iCloud and sign in with your current iCloud ID and password.
If you have any photo stream photos on your device that you want to keep, save them to the camera roll before deleting the account.