The 20th Anniversary Edition has the same mix as the original P&S edition. I know this because I made a tape recording of the VHS tape released at the same time as that original LD, and it was the same mix as on the THX LD box set.
The 5.1 sound mix that's on the DVD was significantly different, but as near as I can tell, that's the same mix that's on the Blu-ray. The DVD was the first time the original mono track was offered as an option on home video.