Q: Change of Apple-ID for iCloud on iPad

You have to sign out of the old ID, provide the password, then sign back in with the new ID.  If your password isn't accepted to sign out of the old ID, go through the steps in this article: If you're asked for the password to your previous Apple ID when signing out of iCloud.  The last one (change your ID temporarily) will usually fix things.

The ID will not automatically change.  You have to go to Settings>iCloud and sign out (or tap delete account if you are not running iOS 8), then sign back in with the new ID.  However, you will have to provide the password in order to sign out of (delete) the old account (ID).  The linked article explains what to do if the password for the old ID is not accepted.