The number is associated with your apple id (the usernames you use to sign into messages and all other apple services), You'll need to change it there first.
My Apple ID
Once you've changed the iphone number associated with your apple id, sign out of messages on your ipad and your iphone.
Sign back into messages on your phone only, with the apple id that you just made changes to.
Then sign into messages on ipad with the same apple id as on iphone.
Using an iPhone 5 and an iPad 2 (both iOS 6.1.3) with the same Apple ID. Signed out and signed in again on both devices. After verification, both device show the phone number associated with the Apple ID and the email address. The iPhone also displayed a message that the iPad now receives iMessages at the registered phone number.
Both are set up to use phone number and email to receive. Both are set up to "send from" the phone number. As far as I can tell, iPhone and iPad are set up exactly alike.
BUT: Sending an iMessage to the email address will cause the message to show up on both phone and iPad. Sending an iMessage to the phone number will only deliver it to the phone.
Turned iMessage off and on a few times, signed in and out a few times (with waits inbetween), reset the iPad. The behavior doesn't change. The message on the phone tells me that the iPad has registered with the iMessage server. But why, oh why, does it still not receive messages?