I just re-read Sam Byrdy's (appropriate name, poor spelling for a golf writer.....) "Alister MacKenzie's Masterpiece - Augusta National Golf Club" again. That should be required reading for anyone answering this thread since it show the original, and dates of all major changes to each hole.
For me the restoration point would vary by hole. How could you not want Maxwell's 1938 green in the design? I think that hole could do without the final lengthening to over 450. I think it was okay to take it to 410 in 2002, though.
Ditto RTJ's version of the 16th, which replaced a 100 yard hole that was too similar to 12 anyway?
Perusing the book and its original photos, it struck me that MacKenzie may have been using too much depression era thinking in using a minimum of bunkers rather than a 'create a masterpiece for the legend of our time" thinking. IMHO, he overused the freak greens (holes 4, 6, 7, 9, had "L" or "U" shapes with very narrow pinning areas, and 8 and 18 were greatly elongated and narrow) to compensate for the idea that there should be fewer bunkers.
Other greens like 2 and 7 were plain dull looking and needed to be changed. Sarazen called it a poor course.
I really don't have much heartburn with most of the changes made over the years, given the look of most of the original greens. I do lament the excessive narrowing and tree planting. Even if they wanted to narrow some landing areas, that would be okay, but I don't think there is any evidence that narrowing the course has kept scores high.
BTW, I understand the next big project - developing the surrounding land they bought as parking has started, and its scheduled to be a three year process, with the new range open in 2010.