An application program stored in a ROM includes a function lookup data structure in which functions called by the application program have identifiers and memory addresses at which the function is located and can be executed. Upon startup, the function lookup data structure is copied to a RAM as a revised lookup data structure and is compared to a revision lookup data structure also written to that RAM or elsewhere. If the revision lookup data structure contains replacement functions having the same function identifiers but new memory addresses, these new memory addresses are written over the existing addresses in the revised lookup data structure for those replacement functions. The application program refers to the revised lookup data structure to find and execute the functions; thus the original application program on the ROM can continue to be used with revised functions.