How to switch to “Sign in with Apple” with an existing account?

Here is something that bugs me with “Sign in with Apple” or with services that are supporting this feature.

The easy and fun part is when you sign up as a new user of a service supporting “Sign in with Apple”. But what if I’m an already signed up user for many years then I want to switch to “Sign in with Apple” without creating a new account? Some service are providing a way to link an Apple ID in order to enable “Sign in with Apple”. But it is the minority. WordPress.com is such an example.

Am I missing something?