"At first, the intention was simply a re-release of ANH only, using new elements struck from the o-neg. But when they had a look at it, they discovered that much of it had "gone pink". Much of the film had been shot on a new Eastman stock that, as it turned out, was unstable (I recall reading that the Jaws negative also suffers from this).
This shows that when the film was originally conformed, original camera negative was cut, and not dupe negative as is sometimes done instead.
Before any other tricks were done, the negative was meticulously cleaned and treated at YCM labs: removing dirt by hand, fixing splices, etc. The pink portions were then duped and fixed photochemically--as far as I've been able to determine, shots that were not being further enhanced with new effects were not scanned and color-corrected digitally (but anything's possible).
A few bits of negative were too far gone and had to be replaced using new neg struck from existing postive(s). One such source was a three-strip Technicolor (yes!) print Lucas had had made back in the day for his personal archive. I know that sounds unlikely as late as 1977 but that's what I came across in my research.
While they were at it, they replaced all the wipes and dissolves: when opticals are made, at least two generations are lost as the shots are duped and rephotographed, so it was determined that redoing them with 1996 duping technology would yield better results, and it did. (This would indicate that the oneg trims of this footage still existed, which is amazing.)
New negatives of these shots were then cut into the negative, replacing the deteriorated portions.
This process inspired Lucas to let ILM warm up for the prequels by creating new material for the re-release. So all the necessary shots (not the entire movie) were scanned in at 2K, the new CG elements added, and were then shot back out to film and cut into the negative. A few 1976/7 special effects shots were scanned in for the sole purpose of cleaning them of printed-in dirt and whatnot; I have b-roll of an ILMer clone-stamping out dirt blobs from Luke training with the lightsaber aboard the Millennium Falcon, a shot that was not plastered with new CG elements.
You see what's happening to the negative all this time? It's being subsumed by new material, bit by bit. Some of it avoidable, some not. Somewhere I have a sound bite of Lucas saying the negative now contains something like 250 (I don't remember the exact number but it's in the 200s) pieces of new negative. This is the reason Lucas has said the original "doesn't exist anymore", because from the perspective of original elements as the ideal, it doesn't. The fact that one could always go back to existing positive elements is of course what made the statement truthful only from a certain point of view."