Yeah, the DS9 episode was upscaled. TNG is being remastered from the original film elements like TOS though.
The problem lies in the effects shots. While they did use models for the ships, the composite shots with all the effects in place were done at standard def. They'd have to recomposite all the original film and re-render the CGI bits to recreate them in HD. I could see that being fairly time-consuming.