Silver-poly (dimethylsiloxane) nanocomposite films are prepared by an in-situ synthesis by incubating the polymer film in a silver nitrate aqueous solution and using the reducing properties of the polymer’s curing agent. Silver nanoparticles concentrated in the surface layer of the silver-poly (dimethylsiloxane) nanocomposite are further replaced by gold through a galvanic replacement reaction at the poly (dimethylsiloxane)/gold salt solution interface. It is demonstrated that, as a result of the galvanic replacement, the quasi-spherical silver nanoparticles uniformly distributed on the surface of the polymer are transformed into ribbon-like elongated gold aggregates. The formation of core-shell Ag-Au nanoparticles as an intermediate state of the low-temperature galvanic replacement is evidenced through microscopic and spectral methods. The collapse of this structure, in a later stage of the replacement, leads to the formation of elongated gold aggregates. The mechanism of the galvanic replacement under different conditions is investigated by closely following the evolution of the morphology, composition, and optical properties of the metal nanoparticles.