Currently My project is using access_token for authorization.
If I move to IdToken for authorization, How will i be able to retain Logins of all my Existing Logged In user i.e maintain session for the existing users which are already logged in using access_Tokens?
This depends on how the back-end logic was implemented for your application.
If you are using the JWTs just to validate the user and then create a session, you can leverage
sub claim will contain only the user’s Okta ID.