When LFL started pushing me in that direction to be honest.
Ironically I first saw the SE theatrically and not the OOT. I rented and bought VHS tapes for years afterwards, and as a kid found myself continually confused as to why the two versions existed and began to compare intensively and realizing that all the new "improvements" and edits weren't necessarily for the better.
In the age of VHS/LD as DVD began to slowly take hold, you had the two editions and nothing else. A DVD release was still far off in the horizon. Then came the ultimate-new films.
I didn't hate TPM. I still don't hate it. It's a well made film on its own that is just slightly dumb in places in its narrative. The technical aspects are great, especially the sound mix and even some of the cinematography is pretty good. But it isn't a great Star Wars movie, and that has been its central problem.
I suppose my real disenchantment began with the opening of AOTC, which I dutifully saw at midnight, and found myself disenchanted with the whole idea of Star Wars. So I saw the film a good 4 or 5 times theatrically afterwards I believe in some vain effort to rediscover a sense of Star Wars in there somewhere. By the time of the Omnimax curved dome screening making me physically sick in addition to the malaise of disenchantment, I knew it was time to put it aside for a while.
But the big one was September 21st 2004, the day of the OT DVD release. I bought this thing, standing in line, reserved-the works only to come home that night and sit through these with my dog only to become horrified at what had been done to not the OOT but the SE itself. The color was all wrong, the sound was wrong, the new edits and sequences were horridly sticking out, and by the time of the Anakin head replacement I was done. Just done.
ROTS sealed the deal for me, as that is probably the 2nd most deflating experience I have ever had in a theater (the no.1 is Casino Royale) because you not only knew EVERY SINGLE THING THAT WOULD HAPPEN. but that these were done in a way that ANYONE COULD HAVE DONE.
From 2005-2008 I gave up Star Wars entirely. For someone who as a child had been an incessant fan, (My childhood was comprised entirely of Film, Literature, James Bond, Star Wars and Batman) this was really hard to do but I just found I didn't care anymore.
In 2008 I started lurking here and finally took the plunge by renting the GOUT disc. And that's where both the admiration and frustration came back. Love for the universe returned, memories of childhood, but all thanks to a shoddily tossed off port of a 1993 LD master with horrid motion smearing, and tinny audio. But it was knowing that I wasn't the only one out there, and that the possibilities of the films still existed that got me excited again, and to be honest that excitement hasn't waned despite the further revisions and now with the big changeover.
But if I had to pinpoint the exact moment where I became a purist, it's not the discovery of so many losing all their hard 1977 work to a CG effect, it's not blinking Ewoks, bad EU, blowing more $ on merchandise or any other of such good reasons....it was rediscovering the spirit of the original film itself, one of discovering one's desired adventure along with a sense of purpose. The moment in the end credits where the score fades to the strings just before the final musical climax brought tears of long repressed joy to my eyes. I couldn't help it. Still can't.
When and Why: The mono mix and Puggo Grande.