Set this up before you need it, not after. The moment your old device is gone is the moment you cannot prepare.
If UIID Cloud is enabled (the default): Your identity is kept as an encrypted backup, so a new device can restore it. Losing a device does not mean losing your identity.
If you turned UIID Cloud off: Your exported .uiid file is your only copy. Have it — and a copy of it — before you wipe or hand over the old device.
Before you switch: Confirm your backup codes are saved somewhere you can reach without the phone, and check which sign-in routes are configured: email, your UIID or .uiid file, or a login certificate.
On the new device: Sign in using one of those routes, then re-enrol biometrics on the new hardware. Your Core ID and aliases come with you; they are not tied to the handset.
Afterwards: Remove the old device from your account and review which apps and agents still hold access.
Stuck mid-move? [email protected]