Again, this paper provides the answer:
"The discovery of the body: human dissection and its cultural contexts in ancient Greece.", by H. van Staden, Yale Journal of Biology and Medicine. 1992 May-Jun; 65(3): 223–241
The main thrust of it is: Greek society, as a whole, didn't change. Most Greeks likely would have found his work mortifying. After Herophilus and Erasistratus, human dissection again became absent from all subsequent Greek science, not to resurface for some 1700 years (in 14th century Italy). However, a unique set of circumstances were at work in Herophilos's home of Alexandria.
One of the key elements: Ptolemaic Egypt (and thus Alexandria), was not democratic:
In Alexandria, a scientist's fellow-residents could not vote to ostracize or exile him on grounds of impiety, as they could--and did--in "democratic" Athens
Further, the Ptolemies, wanted to establish Alexandria as a center of learning, and so extended generous patronage to scientists and scholars. It was the kings who, not only allowed Herophilos and Erasistratus to pursue cadaver dissection, but also handed over condemned criminals for vivisectory experiments.
The Ptolmies also had little respect for the Greek taboos of the time, and may have set examples of violating them, themselves:
Moreover, Alexandria was a new frontier city in which traditional Greek values were not considered intrinsically superior. Indeed, the Ptolemies themselves set examples--shocking to some Alexandrian Greeks--of violating traditional Greek taboos...
Also, the philosophical climate may have contributed making it more acceptable, particularly Aristotelian thought, who spoke specifically on the subject of the corpse:
It is clear that a corpse is a human being in name only...
Even though a dead person also has the same external form and shape, it nevertheless is not a human being...
So too the "hand" of a person who has died is a hand in name only, just as flutes in a stone sculpture might also be called "flutes"...