The easiest way to change the function would be to use 3 external PFETs; currently it's a low-side switch, with the gate voltage going above ground to turn the FET on and let current flow from D to S.

To change this to a high-side driver (enabling the current flow at the supply voltage side), you'd take a PFET and wire it like this:

- Source of PFET to LED power supply (body diode cathode points to power supply)
- Gate of PFET connected to PFET Source with a resistor - say 10k or 100k
- Drain of PFET connected to R/G/B LED string power line

...then, connect the gate of the PFET to the appropriate R/G/B of your power control board.

The PFET will turn on when the gate drops below the source by >Vgs. When the lighting controller NFET is off, the PFET gate is pulled up to the source by your resistor - ie, no current will flow.

When the NFET in the LED controller turns on, the gate of the PFET is pulled low, meaning Vgs goes negative (Vgate < Vsource, as Vgate will be near ground and Vsource will be your supply voltage). Current will then flow and the LED will light.

Ok, so you'll need to find yourself 3 PFETs (at least 20V rated; getting 60V ones would be best for robustness), but you should be able to do this all "in the wiring" as it's just a single FET and a single resistor per channel. You can also try just one channel to see how it works.