The most popular mod is the LED swap. It's usually done to get a different color temperature (yellow/blue balance), tint (red/green balance), color rendering index (whether the full range of colors appears natural or not), or beam shape (hotspot size - whether the light is better suited for use up close or far away).

It's fairly easy to do if you know how to solder, and especially easy if the LED is purchased already mounted to an appropriate metal-core printed circuit board. Just desolder the leads, remove the old MCPCB, put some fresh thermal paste on the bottom of the new MCPCB and solder the two wires to the contacts.

The mods you see here go beyond that. These lights have third-party drivers which contain a microcontroller, power regulation, and separate circuits for the colored secondary lights you see. They're probably this driver. They also have special MCPCBs with pads for the secondary LEDs, and some have aftermarket optics.