The edited masters were all 4:3 SD, at least until the early 2000s when HD became prevalent, and were all mastered to the highest levels of quality available at the time. To truly re-master to higher quality would involve telecine/scanning of the film, if shot on film, and completely re-editing and this is unlikely at best.
Perhaps they are re-sampling, upscaling, enhancing color and filtering to get rid of NTSC color artefacts, etc., contained in the original edited masters but they are almost certainly not re-mastering using the original sources or even submasters (such as might be done when re-mastering audio from the original multitrack master).
Here is the remastered Petty Free Fallin' and although it is presented at 1080 it is still 4:3 aspect and is clearly not at HD resolution; it looks very soft by today's standards.