To migrate your site to use Identity Toolkit v3 for authentication, it is possible to maintain full availability by first enabling dual write for any changes. This means if a new user signs up, the record is created in both your existing database and the Identity Toolkit v3 database.
Then in the next step, you will upload all existing users to Identity Toolkit v3. This will mean that both databases run in parallel and in sync. Then the last step will be to implement the Identity Toolkit v3 widgets for your site before you can fully turn off the previous authentication system.
Implementing dual write
Here you will modify the existing logic in your new account flow to dual-write to your local account database and to the Identity Toolkit back end. This will be achieved with the Upload Account API
You will need to set up a Service Account (as described in the Configure Service step) and use the uploadAccount API to write entries to Identity Toolkit. This is easiest if done by using the per-language libraries included with the quick-start apps. Each includes a README file to describe how to upload accounts in that language.
You will need to write these entries to each database serially, first into your local account database and retrieve your localId and then push this value to Identity Toolkit to maintain a consistent key. You need to set up this dual-writing system regardless of the method new users are signing up for your app: web or mobile, password or IDP sign-ups. Keep track of what time you have this fully rolled out to help you understand how to manage the user upload process.