Why did Adafruit pick a 10K resistor for this phototransistor breakout?
The datasheet says that the


  loading resistor shall be chosen to meet the requirement of maximum ambient light, and output saturation voltage


However, Adafruit cannot know what ambient light level and supply voltage you will be using, so the resistor is somewhat random.
If it's too small for you, put another one in series; it it's too large, solder another one in parallel, or replace it.

You can switch between multiple resistors at the same phototransistor. Current flows only if the other end of the resistor is connected somewhere, so route the resistors to multiple GPIOs, and switch those between ground (low-level output) and high impedance (input).