I do NOT recommend changing the LED. The voltage drop across the LED provides the reference voltage (called the "band-gap" voltage) for the current source. That voltage drop is specific to the LED chemical formulation; the one we use is "standard red" which is almost extinct now, replaced with higher efficiency light emitters. The more efficient ones provide a less stable voltage, as well as a different voltage. Email Eileen and she'll send you some new LEDs of the right type.
