As noted, it seemed to be less of a sudden change and more of a gradual shift.
I've got a 1974 Raleigh catalog as well as a 1976 Raleigh catalog. In the '74 catalog, all of the bikes with dropped bars were sporting high flange hubs. Here's the page for the Team Professional, the top of the line bike:
By 1976, the Team Professional was fitted with small flange hubs, while the remainder of the line-up were shown with high flange. To me, this suggests that small flange were the more desirable version.
I'll admit that I started building my wheels with small flange hubs at this time too.... I guess I'm just a sucker for good marketing (as evidenced by the fact that I currently own a Team Pro)
Steve in Peoria