How to migrate users from AspNet Membership to Okta

I need to replace Asp.Net Identity with Okta in an app that runs on .Net Framework 4.8. Can I have some guidance on the approach that should be taken to perform the migration gradually?
My understanding is, after registering the app in Okta, all unauthorized requests will be redirected to Okta’s login screen. However, I need to migrate the users gradually, what should be the migration path? I also want to expire the passwords of migrated users to make user that every user is following a certain password policy.