If you wish to change and use an entirely different AppleID, first ensure that you have disabled the Activation Lock that will otherwise cause difficulties. You can disable the Activation Lock by disabling the Find My iPad service from iPad settings before trying to use a different AppleID account:
Settings > [Your Name / AppleID] > Find My > Find My iPad - set to OFF
Having disabled the Activation lock, you can now sign-out from your old AppleID:
Settings > [Your Name / AppleID] > Sign Out
Presumably you’ll want to keep any iCloud data that is locally stored on your iPad for transfer to the new AppleID. If so, accept the opportunity to keep a local copy of your data (e.g., Calendar, Contacts, Notes, Keychain Passwords etc.) - as otherwise this data will be wiped from your iPad as you sign-out from iCloud. If you don’t want this data to be merged with the new AppleID account, do not opt to keep a local copy of data.
Also ensure that you have signed-out from Messages and FaceTime:
- Settings > Messages > Send and Receive > [AppleID] - scroll to the bottom where you will find your signed-in AppleID highlighted in blue text; tap and select Sign Out
- Settings > FaceTime > [AppleID] - again, scroll down the page to find your signed-in AppleID highlighted in blue text; tap and select Sign Out
After signing out, a restart of your iPad is recommended:
Restart your iPad - Apple Support
After restarting, you can sign-in to your new AppleID account from iPad settings - and also sign-in to Messages and FaceTime if necessary. Any iCloud data that you saved to your iPad during the previous sign-out will be merged with your new AppleID account.