As I understand currently you are using OKTA as identity provider to sync users and their attributes to Azure AD.
And you want to integrate your new IDP with Azure AD for your custom domain.
IDP is used for user authentications. And your question is regarding users getting provisioned using new IDP. To sync users to Azure AD you need to use the Sync tool.
Once you disconnect OKTA, it will break the link between users and OKTA in Azure AD. However, while installing new sync tool, the users should get soft matched and there should not be any duplicate accounts created.
When you switch the Identity Providers, there is a change only in the authentication authority.
Let me know if you have any further questions.
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