Migrating Off Homegrown SSO Without Logging Everyone Out

Replacing a homegrown SSO system is chiefly a coordination and identity-continuity challenge rather than a protocol implementation task, especially because changing an SP Entity ID or ACS URL requires every customer to update its identity-provider configuration. A safe migration should first inventory all active connections from observed authentication traffic, including undocumented tenant-specific behavior, certificates, attributes, bindings, NameID formats, and last successful logins. Existing user mappings are particularly important for persistent SAML NameIDs, which are pairwise opaque identifiers that may change if the service provider identity changes, whereas email-based identifiers are generally easier to map. The recommended approach is to preserve external identifiers and session behavior, operate legacy and replacement authentication paths concurrently, validate the new path in shadow mode using real assertions, and route tenants through a data-driven per-tenant flag that allows immediate rollback without deployment. Tenants should be migrated individually, beginning with internal or smaller cooperative customers and monitoring login success before proceeding, while the legacy path and non-destructive schema compatibility remain in place until all customers have been stable for a meaningful period. Customer communication should be transparent, particularly with security contacts, and any unavoidable configuration changes should be separately planned with clear instructions and agreed change windows.

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