They were shot on negative 35mm film, and edited on tape in that aspect ratio.
Only some odd scenes and shots were captured on tape, but it was mostly film. They won’t need to upscale any live scenes, but they would have to work on rendering all the digital effects and blue screen shots for example. As it was only ever edited on tape, it’s unlikely that the digital effects could be ever rendered or upscaled, it would probably need to be entirely reworked from scratch.
Very daunting task. Although, they did it for TNG. ¯\_(ツ)_/¯
Edit: Someone pointed out the obvious in a reply. Yes, of course they will need to be scanned for a better resolution scan. I already pointed out it’s on film, and even mentioned why it’s not just that simple. (Hence the digital effects needing to be redone).
*cries behind my Ray-Bans"