This week ABC will quietly revolutionize its app for iPhones and iPads with a button called “live.” Users around New York and Philadelphia will be able to live stream all the programming from ABC’s local stations there, the first time that any major broadcaster has turned on such a technology. The functionality will be featured at ABC’s upfront presentation for advertisers on Tuesday. It is, among other things, an attempt to keep up with the rapidly changing expectations of television viewers. It also reflects the increasing role that subscriber fees play in the broadcasting business: the live stream will be available only to paying subscribers of cable and satellite providers, even though the stations’ signals are available free over the public airwaves. ABC, a unit of the Walt Disney Company, said the live stream would be available in the other six cities where it owns stations sometime this summer.