Before upgrading, back up your project to avoid any data loss.
APIs marked as deprecated on 1.2 and earlier are removed on 1.4. Make sure your project is not using them before upgrading.
AuthClient API changes
We've greatly simplified the AuthClient API which should make it much easier to use and much harder to get wrong.
Part of the changes is moving the responsibility of storing the guest login credentials to the consumer of the AuthClient.
All Login methods return a LoginResult object which contains credentials required for further sign-ins, including the SessionToken which was previously stored in the AuthClient. It is in caller's best interest to store those credentials so they can be reuse in the next session.