I’m not sure WHEN Lucasfilm digitized the “original master tapes” (if that’s what they used)?
I’m GUESSING it was around the time (may be partly because of) the 1993 LD box set was released (also the 1993 CD Original Anthology soundtrack box set): both of these were pretty big production efforts for Lucasfilm; further, Lucasfilm had the SE projects in the plans .So it makes sense that they “invested” in the soundtrack at this time.
Magnetic tape (with analog audio or video) degrades – OVER TIME – for several reasons (tape plastics disintegrate, magnetic loss, Earth’s own magnetic field playing havoc, etc).
The optical (audio) film tracks do not have the rez/fidelity of analog/magnetic tracks (much less the orig. tapes), but they (optical audio) don’t suffer from de-magnetization (= fidelity loss) or print-thru (= pre- or post-echo).
Bottom line: One can only hope the original tapes were digitized in the early 1990s (if not earlier).