No idea when it changed but the reason why the last tone is longer is very logical.
The long last tone signifies that that is the PRECISE stroke of the hour - very exact although, paradoxically, latest developments in digital and satellite broadcasting (and the time lag) make this less accurate than i was a few decades ago.
Just swanned round Wiki which is very good on the early history of the pips but strangely silent on the seventh pip.
Wiki says (among other interesting things)
"The pips have been broadcast daily since 5 February 1924, and were the idea of the Astronomer Royal, Sir Frank Watson Dyson, and the head of the BBC, John Reith. The pips were originally controlled by two mechanical clocks located in the Royal Greenwich Observatory that had electrical contacts attached to their pendulums. Two clocks were used in case of a breakdown. These sent a signal each second to the BBC, which converted them to the audible oscillatory tone broadcast.
The tone on the line was inverted; that is to say, the signal sent to the BBC was "on" when no pip was required, and was pulsed "off" when a pip should be sounded. This allowed a fault on the line to be detected immediately."