The Karma Kaanda of the Veda-s state how a Braahmana cannot be a Braahmana without eating beef. There were rituals that required the best bulls to be killed whenever a Sannyasi or king visited one’s house. But then it was found that since we are an agricultural race, killing the best bulls will lead to the extinction of the species. So the ceremony was abandoned.
Swaami Vivekananda also said that the Buddhists introduced many external reforms into the country, and Shankara Aachaarya had to undo all of them by emphasizing the spiritual portion of the Veda-s (by his commentary on the Brahma Soothra-s).
Likewise, the Smrithi-s would change by the times, although the Shruthi-s would remain constant.