Migrating existing users to a new centralized user datastore after switching to a modern identity provider is crucial. Centralizing user accounts offers several benefits, including having a single place for all user data management, reduced complexity in managing access and authentication, and improved security through a unified authorization logic. However, migrating existing users without impacting their ability to use the application poses significant challenges, such as ensuring minimal downtime and minimizing the risk of errors during the migration process. Two common approaches to migrating existing users are the "big bang" approach, where all users are migrated at once, and the phased migration approach, where users are migrated one at a time through authentication. The big bang approach is simpler but carries more risks, such as missed testing or unexpected errors, whereas the phased migration approach is slower but offers reduced risk of issues during the migration process. To facilitate phased migrations, developers can use FusionAuth Connectors to build an HTTP JSON API endpoint that receives user credentials and returns a JSON response, which can be used to authenticate users and migrate their data to the new centralized user datastore.