I don't see why not. When season six of TNG-HD comes, we'll get something of a preview of DS9 in HD with "Birthright, Part I."
See posts above for why not. Birthright
won't be a problem as, AFAIK, there was no CG in that ep - shots in and outside the station were probably filmed on 35mm. But there's an awful lot of CG in the series itself, which'll either have to be remade (which will cost them) or upscaled (which'll look like crap).