This?

Change phone number on the same phone
https://faq.whatsapp.com/498754122134583/?c...latform=android

You will be given an option to notify your contacts about the numbers change.

However all groups you are in will be informed regardless, so you will need to first leave those that you don't wish for it to be known.

Imagine one day Whatsapp server kongkek

So many ppl saved chats will gone

wont as it is not saved in the cloud or server. It is locally saved in your own device.

Look like you never explorer on phone if is android, iphone i dunno
All chat are store on main device android/medial/com.whatsapp/whatsapp

That's all great until u try to sync to Whatsapp server and chats lost

U know this happens right

There are cases of ppl losing Whatsapp chat history even when backed up

What? When we got sync msg with whatsapp? Once msg send to phone, even whatsapp down, chat will not missing, you still can view on your phone

That is also not related to backup, unless your phone suddenly cannot on, mean all chat gone