What master does the Disney+ version use? Is it the same as the 2011 master (or the 2011 master but upscaled)? Is it the 2011 master but at higher resolution? Or is it a new master entirely?
2011
Incorrect. It is a new tansfer of ANH, TESB, and ROTJ with different colors and a few differences. The changes cannot come from being recolored as they restore previously missing colors. I think TPM is as well and TFA likely is nearly identical, but unlike the Blu-Ray master (which had burned in English subtitles for all regions and languages) it has a clean image and you can turn the subtitles off. AOTC and ROTS have no color changes (makes sense since the digital file is the original image, not a film scan) and is likely upscaled to 4k.