Changes in the shape and size of Co, Pt and Au nanoparticles induced by swift heavy-ion irradiation (SHII) have been characterized using a combination of transmission electron microscopy, small-angle x-ray scattering and x-ray absorption near-edge structure. Elemental nanoparticles of diameters 2-15 nm were first formed in amorphous SiO2 by ion implantation and thermal annealing and then irradiated at room temperature with 27-185 MeV Au ions as a function of fluence. Spherical nanoparticles below a minimum diameter (~4-7 nm) remained spherical under SHII. Initially, they slightly increased in diameter but thereafter were progressively dissolved into the SiO2 matrix. Spherical nanoparticles above the minimum diameter were transformed to elongated rods aligned with the ion beam direction. The nanorod width saturated at an electronic-energy-deposition dependent value, progressively increasing from 4-6 to 7-10 nm (at 5 to 18 keV/nm, respectively) while the nanorod length exhibited a broad distribution consistent with that of the unirradiated spherical nanoparticles. The threshold diameter for spherical nanoparticle elongation was comparable to the saturation value of nanorod width. We correlate this saturation value with the diameter of the molten track induced in amorphous SiO2 by SHII. In summary, changes in nanoparticle shape and size are primarily governed by the SHII parameters not the metal nanoparticle properties.