Hello Roderick,
I have had a similar issue before, and this is my experience:
1st of all, when you backup app data to iCloud (or any other similar service/OS), when you restore data, it will restore from that backup.
So, even if you change numbers, you get the previous data restored to your "new" phone/phone number.
What you have to do is this:
There can only be 1 (one) WhatsApp backup, so you have to decide if you want to keep your old backup, or start fresh and NOT back up the new phone.
Under:
>Settings > Apple ID > iCloud > WhatsApp
you can control this by opting to backup/restore from iCloud, or not.
The same principal applies to all other app data.
Good luck.