If your iPad is signed into an old iCloud account ("email address") and is prompting you for the password for it, you change the account to your current ID. To change the iCloud ID you have to go to Settings>iCloud, tap Delete Account, provide the password for the old ID when prompted, then sign back in with the ID you wish to use. If you don't know the password for your old ID, or if it isn't accepted, 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 iPhone on your device, even though it prompts you for the password for your old account ID. Then save any photo stream photos that you wish to keep to your camera roll. When finished go to Settings>iCloud, tap Delete Account and choose Delete from My iDevice when prompted (your iCloud data will still be in iCloud). Next, 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.