Adding up the costs of a diy cnc machine continues to make my eyes water so finding these makes for a refreshing change :)

In theory the output from the opto isolator should be wired to a feedback pin on the main PWM controller chip. If memory serves me correctly and the chip follows convention then the function of this feedback pin is very simple:

if the voltage on this pin is above a certain threshold value (around 2.5v perhaps) the PWM controller switches off and if the voltage on this pin is below said threshold the PWM controller switches on - this is how the output voltage is regulated.

As the PSU already has some way of varying the output voltage there may be some additional complications but all that should be needed to decrease the output voltage is a change to a single resistor such that the voltage on the feedback pin is increased by about 20% for a 40v to 47v output range (ish).

Disclaimer: I know enough to hurt myself and nothing more :)