I can't vouch for the flying qualities, and any tailless design is considerable effort in learning, above and beyond that required for a more conventional configuration. Trust me on this. However, it can be done, and designing an airframe that can accomodate one set of wings and tails is probably orders of magnitude easier than trying to design one that can fly effectively and safely with radically differing performance qualities, flying surfaces, and structures. In the time it takes you to adequately design and build the "ultralight" version of the airplane, you could have your pilot's license and quite a few flight hours under your belt. If you want an ultralight, build an ultralight. If you want a higher-performance airplane, do that. I can certainly understand the mental temptation to try such a 'morphing' model, and your reasoning behind it, but transitioning the same airframe from one realm to another is going to be a bigger challenge, IMHO, than even designing a tailless airplane in the first place.