Migrate Google+ auth to newer Google auth on a Node/Angular app

A popular web app of mine ([login to view URL]) was started many years ago, and uses Google+ and GitHub for OAuth authentication, with Node Express and Passport. With Google+ being shut down, I need to migrate that authentication option over to Google's new version, without existing users losing access to their documents. I hope that the Google+ and new Google Identity "sub" property on authenticated users are identical, in which case that won't be an issue. I am looking for a developer who's experienced with Node and Javascript, and OAuth based authentication, to execute this migration for me. It will involve using Google's new authentication client library ([login to view URL], [login to view URL]), and adapting the Node backend to validate tokens from that and return relevant user objects. The goal is that nothing much changes from a user's perspective, other than seeing a new login button and the new OAuth screens.