It would be way too expensive to do it and make it way more expensive than TNG which is not a good idea when the spinoffs where not as popular as TNG. I think it would require another company remaking the effects shots in addition to two companies editing them together. Surely this is too expensive. Its not just re-rendering the CGI models (which would have to be remade from scratch!) they have to match the virtual camera movements and positions of ships etc. Imagine trying to remake the fleet scenes in the latter seasons of DS9 - it would cost a fortune! OTOH they can simply upscale. Enterprise already has 480p CGI upscaled and so will DS9 and VOY. Surely the VFX companies sent in their CGI effects in a higher bitrate ready to be edited into their SD broadcasts just incase they are upscaled in the future. Perhaps these reels exist separately to the finished episodes, especially if they were edited into existing live action footage.
A. now you see why CBS has been reluctant to commit to DS9 or VOY
B. You way overestimate the CG that was done at the time. It will need to re-done. Remember the CG at the time was done as low resolution and with as low geometry as possible for 90's standard definition TV. It will look like just terrible when upscaled. You can already see low resolution textures on Voyager if you look.
Both Adam "Mojo" Lebowitz and Doug Drexler have said basically as much at different points in time when asked on their respective blogs.
was done with much higher resolution initial models, etc. but rendered at 480p for rendering speed given the budgets and weekly schedules. It was eventually rendered at 720p depending on the shots.