In chapter 8 of Nectar in a Sieve, how does the tannery change the lives of the four women who met so long ago?

The tannery has indeed changed the lives of the four women. Although three of the women lament the changes, Kunthi is positively inclined towards the tannery. She is appreciative of the economic benefits it brings to the village. As an example, Kunthi tells Rukmani and Janaki that her two eldest sons already bring home more than a man's wages from working at the tannery.
Kunthi is also pleased that the tannery has apparently changed their dreary village into a growing town. She is simultaneously ecstatic that this change is accompanied by the appearance of droves of young men to the village. Kunthi delights in going into town, "where, with her good looks and provocative body, she could be sure of admiration, and more, from the young men."
Meanwhile, Janaki is less than enthused with Kunthi's attitude. She frets that her husband's shop is struggling to compete against the newer, bigger shops in town. Eventually, Janaki and her family have to leave the village in order to search for new opportunities elsewhere. Although Rukmani is sad about this new development, she is forced (by circumstances) to refrain from dwelling on her feelings.
Rukmani and her husband are mainly small-scale farmers, and they must continue to survive even as the economic landscape in their village changes. Both Rukmani and Kali envy the Muslim wives of the men who work as tannery officials. These wives have servants, and they don't appear to have to work for their sustenance. Rukmani rejects such a life; despite the challenges inherent in farming, she would rather revel in the "open fields and the sky and the unfettered sight of the sun" than submit herself to a life of lonely seclusion.
From the above, we can see how the tannery has changed the lives of the four women. While Kunthi embraces the benefits the tannery brings, Janaki's life has been destroyed by the tannery. Her family has had to relocate and invent new opportunities in response to the changes. Meanwhile, Kali and Rukmani choose to stay, but their lives are made more difficult as a result of their decision.