It is common for industrial installations to be in operation for several decades, which means that upgrading the control systems of these plants is an imperative for several reasons. As new hardware and software solutions become available, spares are increasingly hard to find and technical support for the old hardware is dropped by the vendors. Lost or non-existing documentation turns usage by new operators needlessly hard and maintenance work or modifications nearly impossible. This paper deals with one such case: the upgrade process of the control system for the Chemical Polishing machine used at CERN for the surface treatment of superconducting RF cavities. It was developed back in 1989, based on Siemens S5 PLCs and a custom HMI. In order to guarantee a smooth and swift transition, CERN's own framework for process control (UNICOS-CPC) was used to develop a new control system, based in current Siemens control solutions. Ideally, this study may serve as a starting point for other engineers dealing with a similar situation.
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