A method for upgrading a portion of a process control system from a legacy programmable logic controller (PLC) to a non-PLC process controller is disclosed. A mounting rack sized to fit into a space occupied by legacy hardware is assembled and populated with replacement hardware that includes an I/O card, an I/O terminal block, and a custom interface module. The custom interface module is coupled to the I/O card via the I/O terminal block and to a plurality of process control field devices via a legacy wiring mechanism coupled to legacy wiring of the process control system, without requiring modification or re-termination of the legacy wiring. The legacy wiring mechanism is disconnected from the legacy hardware, the rack containing the legacy hardware is removed and replaced with the mounting rack, and the legacy wiring mechanism is coupled to the custom interface module.