iphone transfer to new phone

Just transferred from iphone 6 to iphone 12. Only after I did this did I realise that I hadn't updated my whats app number (from previous country I was living in) to my new number. I proceeded to do this but there has been no notification to all my contacts, especially whats ap, of the change in mobile number. On transferring phones I also seem to have lost the last three weeks of whats app messages.Anyone know why this has happened and how to fix it?