Does anyone have clues about how to do this? I'm basically trying to replace the strategy for "Connect With Google" from OpenID to OAuth. The challenge is identifying an old user (user on Google open ID) when a user signs in under the new OAuth scheme.
I have a working implementation which relies on email address as the primary key, as the open ID strategy captures that. The problem is, I don't want to ask for email in the OAuth flow. The ideal value is simply Google user ID, but the Open ID strategy doesn't seem to capture that.
So I have open ID tokens like https://www.google.com/accounts/o8/id?id=AfSCwGQ4PUaidXSQddJugXKLqU5V0MrXFhJM6UHybPw and trying to understand if I could get a Google ID from that.
UPDATE: I explained here how I ended up doing this migration - http://softwareas.com/migrating-user-accounts-from-google-openid-to-google-oauth-to-google-plus
Google uses directed identifiers for OpenID that are unique per relying party and are explicitly designed to conceal any correlatable identifier for the user. So the short answer is, no there's no way to get a Google ID that corresponds with a given Google OpenID.
One option, however, might be to use Google's OpenID+OAuth Hybrid flow. This allows you to get an OAuth token as part of a normal OpenID flow, which could then be used to get the user's ID from the OAuth2 Login API, which you can then associate with their existing account. Once you've done that for all of your existing users, then switch to using the OAuth2 Login directly.
The trick, of course, with this approach is getting all of your users to login again so that you can send them through the new flow. That will come down to how long you're willing to wait to migrate accounts, and whether you're willing to prod existing users by emailing them and asking them to login again (similar to a forced password reset).