How to transfer WhatsApp chat from old to new number without losing data?

Install WhatsApp in the new phone and login with the new number. When WhatsApp will look for backup, tap on ‘Ristore’.

This will return all your data (chats, groups, photos, files, everything). If you want to setup from the old number first, then want to enter a new number, then restore the backup first and then add a new number to the ‘Change Number’ option.

In this way everything will be organized and no information will be missed.