The unabridged version I just finished ends with the following words:
February 1975
December 1988
So, he apparently had been working on it in 1974, if not even earlier.
The version begins with an author's note in two parts; one is a very funny preface to be read before purchase, and the other a preface-part II, to be read after purchase. He explains that this edition adds no new characters, just fleshes them out more. One example is that it includes a confrontation between Frannie and her mother that apparently didn't make it into the original release, or at least not in the same detail.
Interestingly, he also offers thoughts on whether The Stand would ever be made into a movie, saying, "probably yes." (In fact, it did become a tv miniseries in 1994.)
As I was reading this version, I too initially wondered whether he had just added back material that had been cut to meet the accountants' demands or had actually added some updated cultural references. It eventually became obvious that he had indeed introduced some updated material. An obvious example was when he referred to HIV and AIDS, which were completely unknown in the 1970s.