Legacy systems rarely need one answer across the entire estate. The right path depends on business criticality, dependency risk, change pressure, and how reliably the system can be separated from the work around it.
Use this checklist to make the decision explicit before implementation begins:
Retain + wrap — Keep the core system where it remains dependable, then add reliable interfaces around it so new capabilities can be introduced without disturbing the system of record.
Strangle — Place a new capability beside the existing one and progressively route responsibility away from the old path as each slice is ready and validated.
Rebuild selected domains — Rebuild only the domains where the current design materially blocks change, while keeping unaffected capabilities stable.
Replatform — Move the application to a better-supported runtime or hosting foundation when the underlying behaviour remains useful but the platform creates avoidable risk.
Retire — Remove a capability when its business role is no longer needed, its data has been safely handled, and the operational dependency has been closed.
The choice should be treated as an architectural decision, not a tooling preference. Map dependencies, define continuity constraints, and agree the validation points before committing to a sequence.
TheiaOne applies the TheiaOne Agentic Delivery Framework as the delivery method: Diagnose → Architect → Build → Validate → Transfer. That creates a controlled path from estate understanding to clean handover.