You should be able to simply sign-out of the old account on the iPad and then sign-in using your new Apple ID. You have to sign out from every category that uses it before signing on using your new Apple ID:
1. Settings/iCloud and tap the red "Sign Out" button at the bottom of the screen.
2. Settings/iTunes and App Store and tap "Apple ID: <old-appleid>" field and then "Sign Out" from pop-up menu.
3. If you have an iCloud email account using the old Apple ID: Settings/Mail, Contacts, Calendars and select the iCloud email account and tap red "Sign Out" button.
Now go back into each of those settings categories and enter your new Apple ID/password and sign in.