Besides the length, what is actually different? Are there new scenes? A lot of the videos can be found in 4K, is it just a change in resolution to 8K and up? Back to the length, is the change just a change to an intro or post video advertisement?
Depending on the studio, it could be a massive improvement on the video quality, for others it could just be slight improvement. It partially depends on the effort the studio puts in for upscaling and/or the original source they have to use. Some studios have additional content they add to the remaster they didn’t include with the original or they may make a change in the intro, either of these would render funscripts for the original useless without a decent amount of re-work.
Historically, remastering means returning to the original source material and processing it to create a new master from which copies are made. That would normally mean recopying or rescanning the original film, processing it to remove defects, and so on.
In the context of something that was already originally a digital video, the most it can mean is reprocessing the original source video (i.e. what came raw from the cameras) to produce something that’s hopefully better quality, using newer encoding or processing techniques.
What actually changes depends on the choices of those doing the reprocessing. Be wary of increased resolutions, as that usually means upscaling, which has a widely variable result in terms of quality. It’s still possible that the source material has a higher resolution than the original released copies, but that’s far from guaranteed.
It’s entirely possible for “remastered” versions of videos to be worse in quality than the original releases - I’ve seen that in the past with WankzVR. So the reality is, you can only tell what the difference is by getting both and doing a comparison, or looking for a review of sorts by someone who has done that. At least for one or two examples - you can probably safely assume the quality difference will be consistent across multiple videos done in the same time period by the same source.
Like what Thanny said, but it’s been a recent trend for scenes that have had content edited for compliance reasons will also often have a Remastered tag. It’s disingenuous.