Are you looking to understand the available strategies for migrating your existing Google login solution to Google+ Sign-in? Well… You’ve certainly come to the right place
I strongly recommend reading it. Or at least skimming it since the social login market is bigger and more complicated than it seems. The following is merely a high-level restatement of the migration guide for people who aren't really sure which of the aforementioned technologies they're using.
If your existing system captures the user's email address using a Google identity solution then you can just:
- migrate to Google+ sign-in
- ask for the email OAuth scope
- fetch the user's email address using one of our recommended approaches
- look up the user in your database by email
- associate them with the existing record that matches that email address since Google guarantees that the email addresses are valid
If your existing system doesn't capture the user's email address then life gets interesting.
If you're sure you're using OAuth1+OpenID2 then you can follow the instructions here: https://developers.google.com/+/api/auth-migration#oid2 which tell you how to fetch your old identifier and find out the equivalent identifier with Google+ Sign. Once that's done you can just associate the new identifier with the existing record and the user can sign-in in future with Google+ Sign-in.
If you're using something else then you can ask the user to sign-in twice: firstly with your existing Google identity solution then with Google+ Sign-in. Now that you have both identities you can associate them in your database. Once a critical mass of your users have gone through this process then you can stop using the legacy identity solution. If you have to use this option then I would also recommend reading Michael Mahemoff's experience report from Player.fm's migration: http://softwareas.com/migrating-user-accounts-from-google-openid-to-google-oauth-to-google-plus since I got the idea from him.