How 4.7-inch secondaries could spread to new cruisers

Would new British cruisers use the dual-purpose 4.7-inch gun as their secondary battery?

Probably: if the 4.7-inch replaced the four-inch and 4.5-inch programmes, new cruisers from roughly 1936 onward would likely be designed around it, though not every older ship would necessarily be converted by 1939.

The alternate policy extends beyond destroyers because the 4.5-inch programme had been intended to replace four-inch cruiser secondaries. A high-angle 4.7-inch weapon with credible surface power would meet the same requirement. New construction from about 1936 onward would probably adopt the common gun, and many older ships could be converted during the prewar years. It remains unlikely that every existing cruiser would complete the change before 1939.