A lack of published retrofit methods that account for specifics of plate fin heat exchangers is a serious problem for industrial projects in which the modernization of such heat exchanger networks is carried out. The author proposes an overall framework for the retrofit of a plate fin heat exchanger network that has been developed and applied for the debottlenecking of a liquefied petroleum gas cold box. The framework covers both levels: single exchanger unit and the exchanger network. This retrofit framework builds on a grassroots design method that has recently been developed and applies a retrofit matrix approach to obtain volume distribution for new operating conditions. Two sets of heat transfer coefficients, those for existing and additional volumes, are used. New volume heat transfer conditions are optimized during the procedure: thus, no pre-assumed coefficients are used, as is the case with previous design and retrofit methods. Pressure drop parameters crucially important for this sort of equipment are also incorporated into the optimization framework.