For migration of a Legacy PLC System in order to make plant control system free from obsolescence, functional and maintenance issue, the following points need to be considered:

1. Deriving details from the existing legacy system including the hardware and software components of the system, existing control logic, IO list, communication methods/ protocols, connection diagram and other relevant documentation
2. Identifying the requirement of migration and design the new system considering potential risks such as compatibility issues with the current components
3. Select the new PLC system that aligns with all your requirements.
4. Migrate control logic and programming from old system to new one and replace the outdated/obsolete hardware with equivalent new ones.
5. Conduct integrated testing to ensure that the system works seamlessly and update the documentation to reflect the changes during migration.

Hot cut over migration in control system means that the migration process from the old control system to the new one will be done without stopping the ongoing critical plant process. The following key steps are involved in the process:

1. Understanding that the existing system and the plant process in detail and identifying the risk factors involved during the hot cut over migration.
2. Setting up a duplicate identical control system with common interlocks and critical loops and with the same functionality as the existing one. Then both the control system with be run concurrently.
3. Testing and verifying the communication and control logic of the new system and compare the result with the existing control system during parallel operations
4. Transiting the control from the old system to new system loop by loop along with removing the duplicated signals from the parallel control system
5. Updating all relevant documentation referring to the changes during the migration.