This only applies to apps that use third party logins. If you roll your own login (and only your own login), then you require zero changes. The "Sign In With

This only applies to apps that use third party logins. If you roll your own login (and only your own login), then you require zero changes. The "Sign In With Apple" implementation, at least on the surface, appears to be a standard OpenID process -- the same thing that Facebook & Google use if you're offering a log-in through those accounts -- so actual dev time should be pretty minimal.