can I just check if you've tried the steps in this Apple Support document? It seems it might answer your question.
The steps laid out there for removing your old phone number are as follows.
On your iPhone, go to Settings > Messages > Send & Receive. Select your Apple ID and tap Sign Out. Then go to Settings > FaceTime, select your Apple ID and tap Sign Out.
If the phone number that you want to remove belongs to an iPhone that you can't access, change your Apple ID password. This removes all phone numbers from your iPad, iPod touch, and Mac. Calls or messages for the removed numbers won't appear on those devices anymore.
The steps to add a new phone number are as follows.
1) Go to Settings > Messages and make sure that iMessage is on. You might need to wait a moment for it to activate.
2) Tap Send & Receive. If you see “Use your Apple ID for iMessage,” tap it and sign in with the same Apple ID that you use on your Mac, iPad, and iPod touch.
3) In the "You can receive iMessages" section, make sure that your phone number and Apple ID are selected.
4) Go to Settings > FaceTime and make sure that your phone number and Apple ID are selected in the "You can be reached at" section.