How do you develop a DCS migration strategy for an aging system?

DCS migration strategy: assessment (current system condition, obsolescence risk, functionality gaps, vendor roadmap), options analysis (like-for-like replacement, hybrid migration, complete rip-and-replace), business case (cost-benefit analysis including downtime, training, efficiency gains), migration approach (big bang vs phased rollout, parallel operation period), I/O strategy (re-use existing wiring through I/O conversion, or replace field wiring), control strategy preservation (document current strategies before migration), testing (FAT, emulation, staged commissioning), and cutover planning (minimize process impact, fallback procedures). Risk management: maintain spare parts for legacy system during transition, staff training, and vendor support agreements. Document lessons learned for future phases.