Retrofitting existing plant equipment to a more energy efficient setup is a promising and sustainable way to cut down on the energy demand and consequently the operating costs of a chemical processing plant. While this opportunity is attractive in terms of low capital expenditure and construction time, it is also highly challenging for the design engineers. Degrees of freedom in grassroot design transform into constraints in the retrofit project and finding an optimal design within the constraints of the existing equipment, requires a sophisticated model-based approach. In this work, such an approach is presented for the retrofit of distillation column sequences to thermally coupled side-stream configurations. Based on this approach, an easy evaluation of retrofit opportunities in terms of investment and operating costs is facilitated. The potential of this retrofit is illustrated in the current contribution by means of a case study, which compares the retrofit option to a fully thermally coupled dividing wall column grassroot design. While the dividing wall column generally allows for higher energy savings, the investment in the retrofit is significantly smaller, which yields a considerably quicker amortization.