Upscale the original VFX elements and put them on HD Enterprise?
I think we need to get used to upscaled CGI if we are going to get Voyager and DS9 on Blu. We are already getting upscaled stuff on the Enterprise blu-rays.
Enterprise HD masters were made that way, they didn't change anything for the Blu-rays. TNG, DS9 and VOY need new masters, so they'll recreate anything that isn't HD, including digital effects.
Incidentally, some of the "Space Baby" and its mother was a physical model, some of it was CG. They'll have to rebuild the CG bits, but the model should hold up ok as is.