I think the "magic doesn't work with tech" change would have happened when technology started to make huge inroads on people's everyday lives.
When did it start? Say the age that the Titanic was launched - when people thought tech was there to make the world a better place. Maybe a bit earlier, when Edison and Tesla set people's imaginations on fire.
WWI is another significant point. Before then the bulk of the population was rural - tech wasn't a huge factor in the lives of those outside the cities. Then people left the farms and villages and were introduced to the technology of war.
Now that I think about it - the turbulent post war period might have been the tipping point. When it went from "magic sometimes affects tech and sometimes it causes milk to spoil" to "magic effects tech".
There's a book - Cider with Rosie - that chronicles the life of a boy growing up in a village in England from WWI to the late 20s, maybe early 30s. In hindsight, the writer notes that he was witnessing the end of a way of life that had endured for about a thousand years.
For example, when he was young a trip to the nearest real town, well, that was a major excursion that was planned by a village committee for months. A once a year thing where everyone took buses (horse drawn ones) and rode for hours just to get there, spent a few hours gawking at the place, then spent hours in the buses traveling back. When he was in his later teens you simply rode the motor coach for an hour or so there, spent the entire day enjoying yourself, and then took the motor coach home. The trip had gone from being something big to just another thing.
Walking culture was dying out then as well. Until cars came around, people would walk for hours to get where they were going. There were walking songs, games, etc - all of which disappeared with the coming of the automobile. You went from walking when you wanted to walk to waiting for the bus that would take you near where you wanted to go.
Yes, the years between the wars brought a massive, wide spread shift in thinking in the west as technology entered people's lives. At the same time you had the Empires shipping tech to the farthest colonies - where in some cases it was seen as another form of magic. It looks like the perfect point for the swing to happen.
So if I was making the call, no real changes in hexing until the 1920s, variable from 1920s until the 1880s, and "milk curdles" stuff before that.
Richard