This technical note introduces a novel model conversion technique for structural dynamic systems that is capable of converting a physically realizable, higher-order model (termed the source model) into a completely different, predefined, physically realizable, lower-order model (termed the target model). Upon the complexion of the conversion, the resulting target model preserves or approximates the dynamic characteristics—the chosen structural frequencies and their mode shapes—of the source model. Mathematically, the procedure to implement the model conversion technique is like a model updating approach, which is fundamentally different from that used in traditional model reduction schemes. A numerical example of converting a five-story two-dimensional frame structure model into a five-story classical/generalized one-dimensional shear-building model is demonstrated.