Old number on my Apple ID

I have tried to find the answer to this question before and have been unable to do so. What happens if you try to change the number without remember the old one? I doubt you'll accomplish what you want, but you might get some information. See Use your mobile phone number as your Apple ID https://support.apple.com/en-us/HT207944#change-appleid. See the heading Change your Apple ID to a different mobile phone number.

You need to change the number on your Apple ID. After the change, allow up to 24 hours for the change to sync. Before trying to make the change, sign out of Messages/FaceTime.

Apple ID - Manage