Why should one go this route?
Microlights were born due to costs, yes. But the cost reduction was related to ignore all the safety bells and whistle of a certified aircraft. Now you ask the cheap way to turn history back? Yes, there are certain microlights being constructed as either microlight or light, but they are few. If you now change microlight to light aircraft you have to do most of the paperworks and reconstruction, which was denied to get the costs down. As every businessman will tell you - changing things later is more expensive as if you had done it from day one.
So, yes, you could do something like proposed, but it ain't cheaper.
And what would you end up? I would love to hear the laughter, when an applicant tells he has 200 hours on a C42. If you go for an old 10k C172 6-cyl they will kind of accept you, but not if you present your hours on a brown-packing-tape flying device.