In short
On the same system, Android to Android or iPhone to iPhone, the chats come along via the backup in Google Drive or iCloud, or directly from device to device via QR code. When switching between Android and iPhone, WhatsApp also transfers the chats directly, for which both phones must be switched on and lying side by side. In every case the old phone number must still be reachable, because a confirmation code arrives by SMS.
Check first: three things
The switch takes a quarter of an hour if three things are right. If one is missing, it takes an evening.
- The backup: in WhatsApp under Settings, Chats, Chat Backup you see when it was last saved. If the date is three weeks old, tap "Back up" now and wait until it is done.
- The number: the SIM or eSIM must work in the new phone or still be in the old one, because the confirmation code arrives by SMS. With a new number, first change it on the old phone under Settings, Account, Change number.
- The storage: a backup with five years of photos and videos is easily 10 gigabytes. That much space must be free in the Google or iCloud account, and both phones are on Wi-Fi and plugged in.
iPhone to iPhone
The simplest case. When setting up the new iPhone, Apple offers Quick Start: place the old phone next to it, point the camera at the pattern, wait. WhatsApp comes along with all chats without you doing anything in the app. Alternatively it runs via the iCloud backup of the whole phone. The route via the chat backup inside WhatsApp is only needed if the old iPhone is no longer there.
Android to Android
Two routes: via the backup in Google Drive, offered when WhatsApp first starts on the new phone, or directly from device to device. For that, open WhatsApp on the old phone, Settings, Chats, Transfer chats, and scan the QR code shown by the new phone with the old one. The direct route is faster and also carries media that was not in the backup.
Between Android and iPhone
For a long time this was impossible, today it works in both directions, but only while setting up the new phone, not afterwards. From Android to iPhone it runs via Apple's "Move to iOS" app, which asks about WhatsApp during the initial setup of the iPhone. From iPhone to Android, Samsung and Google offer the move by cable during setup, and WhatsApp then shows a QR code.
Chats, photos, videos and voice messages come along. The call history and display name do not. If the new phone is already set up, it has to be reset once for this, which is the point where most people give up.
Where it usually fails
The encrypted backup: if you switched it on, you need the password or the 64-digit key, otherwise the backup is worthless. The old number: if the contract is already cancelled, no code arrives and WhatsApp cannot sign in. The outdated backup: everything written after the last save is missing. And the full storage that leaves the restore stuck at 80 percent.