Abstract: Methods and tools for implanting prosthetic heart valves and modifying leaflets of an existing valvular structure in a subject are disclosed herein. Prior to or during implantation of the prosthetic heart valve within the existing valvular structure, each tool can be provided in the ascending aorta (or equivalent thereof) of a subject and can be used to pierce, lacerate, slice, tear, cut or otherwise modify a leaflet or commissure of the existing valvular structure. The existing valvular structure can be a native aortic valve or other native heart valve, or a previously-implanted prosthetic heart valve. The modification can avoid, or at least reduce the likelihood of, issues that leaflets of the existing valvular structure might otherwise cause once the prosthetic heart valve has been fully installed, for example, obstruction of blood flow to the coronary arteries and/or improper valve mounting due to a non-circular cross-section.