The HD version is available for rental/purchase on streaming services like Amazon and Google play, and I'm sure other places as well. I'd be shocked if it wasn't HD on Disney+.

(you can watch a segment of the HD 1080 version on Amazon here by clicking on "watch trailer"... it might take a few seconds to buffer to 1080: https://www.amazon.com/Black-Cauldron-John-Huston/dp/B006RZZ80O)