Hi @Corwin, Don
Thank you for posting your question in the Microsoft Q&A forum.
Based on the information you have provided, looks like you want to know if users will be affected during the migration.
From Microsoft documents, the impact on users should be minimal.
This is because during the migration, there's a gradual transition from Legacy policy to Authentication methods policy.
After migration, users should still be able to use Single Sign-On (SSO) as long as the necessary authentication methods for SSO are enabled within the new policy.
You can read more at Manage authentication methods - Microsoft Entra ID | Microsoft Learn
I hope this information helps.
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