[apache]

Well, I think thats a really interesting procedure myself. I also think that it is something that should not be able to be done. I am not saying that you should not be able to retrofit your way to a ring/sphere world, or anything else for that matter, that is perfectly logical and feasible.
However, the real problem here is that retrofits are instantaneous. With retrofits, you can produce components with more total cost than a shipyard can actually produce in one turn. I think that retrofitting a vessel should take time, not be instantaneous. Perhaps that time should be based on the time it takes to make those new components or the difference of cost between the old components and the new components.
Either way, retrofitting should just not be instantaneous, especially if its a starbase undergoing a massive retrofit or something. If some sort of system were put into place that cost more time for bigger refits, then sphere/ring worlds and other things could not be built faster than normal.