Philosophers interested in conceptual engineering take it for granted that the same concept can unproblematically play diverse functions, but this view overlooks the fact that conceptual and functional change often impair concepts and even functions themselves. I demonstrate that while conceptual and functional engineering may improve concepts and functions, they can also produce detrimental effects. Therefore, it is crucial to carefully assess the potential benefits or problems before making any modifications. Frequently, we overlook the fact that, for instance, adding extra (...) functions to our concepts modifies them; this may increase, but also impair, their theoretical and practical efficacy. I analyze and clarify these possibilities through a general classificatory framework encompassing concepts, functions, and conceptual and functional change. The larger aim of this paper is to bring attention to these complex and under-researched relationships and pave the way for further research in this area. (shrink)