The FIDO Alliance (who wrote the WebAuthn spec with the W3C) has a draft specification for a format (Credential Exchange Format) and protocol (Credential Exchange Protocol) for migrating passkeys and other credentials [1]. I don't think this is implemented by any providers yet, but it's being worked on.
conradev|9 months ago
https://github.com/fido-alliance/credential-exchange-feedbac...
I am worried about providers blocking exports “because security”. That’s Apple’s favorite argument