Hi,
I've just made a demo site for Passkey Upgrade (which intoduced by Apple at WWDC 2024)
1. Sign-up by submitting non-registered username & password (and store it to your password manager)
2. Sign-in by submitting already-registered username & password using password manager.
3. Then Passkey Upgrade is triggered automatically, and after that, you can also sing-in w/ Passkey Autofill.
Of course, you need iOS / iPadOS / macOS latest developer beta.
ciao
ref. Passkey Upgrade session at WWDC 2024