You should ask Flight Design, but I would expect that going from a 100hp S to a 135 hp iS would at least require redesign of the engine mounts, cowls and fuel system, and probably strengthening of the airframe as well. Also the 915 weighs about 185 lbs and the 912 weighs only about 125 lbs, which will have a major effect on weight and balance, and reduce useful load capacity. The extra power would also make it much easier to exceed Vne. There is also the question of who could carry out such a modification legally. In summary, it is probably possible, but would be difficult unless you have the resources and expertise of the Flight Design factory engineers.
rgmwa