The first question after buying a new phone is usually the same: "will my old WhatsApp conversations come across?" They will — but not on their own. There is one job you have to do while the old phone is still in your hands. Skip the backup or the transfer, open WhatsApp on the new phone, and your chats start empty with no easy way back.
This guide separates the three scenarios: Android to Android, iPhone to iPhone, and changing platforms (Android to iPhone or back). One condition is shared by all of them: the same phone number. The steps were verified against the WhatsApp Help Center plus Google and Apple support pages; menu names may differ slightly depending on your app version.