How do you approach PLC system migration from legacy to modern platforms?

Legacy PLC migration approach: assess existing system (document I/O, logic, communications, issues), define requirements for new system (functionality, performance, expansion), select target platform (compatibility, support, features), develop migration strategy (big bang vs phased vs parallel). Conversion: I/O mapping (may require new I/O types), program conversion (manual review even with auto-conversion tools), communication updates (modern protocols), and HMI/SCADA updates. Testing: simulate with hardware-in-loop, parallel operation if possible, comprehensive FAT and SAT. Address obsolescence while preserving proven logic. Train maintenance staff. Document lessons learned.