I don’t think the differences I’m seeing are simply due to dnr and blooming. It doesn’t explain why the lightsaber gets completely blown out but the other highlights in the frame do not. For example, the tip/point of the lightsaber in this image doesn’t change, but the entire glow does:

The lightsaber effect is an optical composite, and it looks to me like they made two versions. They made one version that they printed on the theatrical prints. Then later they did some telecine transfers (probably for promotion of the special edition) and weren’t satisfied with how the telecine looked and did a re-composite of the effect for telecine where they gave a greater intensity to the effect, resulting in the blade becoming much brighter. I’m quite certain this is what has happened with the lightsaber rotoscoping - it’s the same effect, just re-composited with a different intensity.

This isn’t that unusual, there are dozens of examples where otherwise “unmodified” films had certain parts recomposed specifically for telecine - for example to make text and credits fit 4:3 better, or to move/resize subtitles. And in fact the subtitles are another thing that was changed between the 1997 SE theatrical prints and the the telecine versions:

So it’s actually not at all unusual they would change this between release formats, but it was previously unknown.