Keyboard mod - Replacing original red LEDs with white
You are right about diode forward voltage. There are three common ways to drive the diodes:


5V with resistor: this is how most indicator diodes are wired. In your case, white LEDs will be less bright but since they are also more efficient, overall light output may be about the same.
3.3V with resistor: white LEDs will not work, but this setup is unlikely as it will put extra load on 3.3v regulator for no reason.
In series with CC LED driver: this is how LEDs are driven in monitor backlights. This provides very uniform brightness but requires more parts. May or may not work depending on the driver IC used: for example, if each chain has 10 LEDs, and driver is rated for 24V max, then this will work with red leds (20v total) but not with white leds (30v total). But if they only put 7 LEDs per chain, then this driver will work.


Overall, I recommend opening up a keyboard and looking at the design. If you see a small IC with inductor next to it, let me know the part number.