To switch to a different phone with a new number while retaining chats and groups, you can either use the Change WhatsApp number feature on your old phone first. Then once you have registered with the new number and created its backup, you can install WhatsApp on the new phone and verify it with the new number. That way, your chats, groups, and other things will be retained on the new phone with the same operating system i.e., Android to Android and iOS to iOS.