On 1/5/2017 11:38 AM, amdx wrote:
....
My question is: why does the diode need
to be replaced?

Because the current limiting is partially determined by the
relationship between the voltage drops of the diode (0.3V) and the
emitter / base junction of the transistor.
If you change only the transistor to (0.6V) you will probably never
develop enough voltage on the transistor to start it conducting.

Mikek

Ah! That makes perfect sense. Thank you.