Yeah been into there it still shows my new number but won't let me change it. The number only changed over this morning so might take a bit of time to change on the phone.
It should automatically switch within 24 hours, a friend had the same problem earlier this week. Be sure and reboot the iPhone every so often and it will force it to recheck the number. It's also worth switching iMessage off/on every so often. Once complete the correct number should display within iMessage settings but also check the Call ID found within Settings/Messages/Receive At/Caller ID.