Welcome to Vodafone Community
You only need to update your phone number.
Settings > phone > my number
I would also recommend using Google for information to update your i message to the new number.
When completed synchronise to itunes/Apple to complete the process and update the SIM.
Go to settings, message, send receive
Tap on Apple ID and sign out.
Turn iMessage off.
Turn your phone off for at least 10 seconds
Go to settings, message, iMessage turn back on
Tap on send receive and sign in to Apple ID
Make sure correct number showin in "start new conversations from"