This paper presents a systematic approach to implementing certain kinds of program modifications, in which (conceptually at least) the modification is implemented as a separate program and then integrated with the original program using semantically based transformations. This approach allows us to ensure that the required modification is implemented correctly and also allows us to explore different ways of implementing a given modification. The approach is illustrated informally using an example where the modification can be implemented in two distinct ways, and then formalised within the refinement calculus by defining a program conjunction operator whose properties justify the transformations required in the example.