On March 23, Candace Browning, the head of global securities research at Merrill Lynch & Co., sent a letter to the firm’s institutional brokerage clients notifying them of big changes coming: Research reports would no longer be available to anyone other than paying clients, and licensing agreements were to be tightened to ensure that Merrill’s work would not be redistributed without the firm’s getting paid for it. The measures, Browning wrote, were designed to protect Merrill’s work from being “Napsterized,” or spread around the Web to a wide array of people who don’t pay for it. Late last month the firm followed by restricting media access to many of the reports generated by its analysts. Merrill’s move underscores a dramatic turnabout on Wall Street. Not long ago brokerage houses cultivated the widest possible audience for their research reports and the highest possible profiles for the people who wrote them. In...