Yeah it's a fine line. It could be argued that the special effects at the time, no matter how dated it may look now, represented the artistic capabilities of the people who worked on it. Especially if that film won awards for them, to re-render later could be argued that it overwrites the award-winning efforts of the time.
Luckily I'm not one of those people.

As long as there are no substantive story/character/dialogue changes (i.e. Greedo shooting first, Bring My Shuttle, etc.) then in general I'm fine with CGI correction/re-rendering. I see it more like removal of matte lines, etc. I would generally not want more special effects (i.e. how Lucas took the Mos Eisley scene and added a bajillion buildings and creatures to it), but if Pixar goes back and simply re-renders the scene to existing HD specs, adding textures, smoothing out polygons, etc. I'm okay with that. Now if they went in and added 50 new toys to the scene...