4 Traces have to be cut. 4 wires soldered in place. Then the original carbon button traces for L1/R1 need to be severed (right in the middle so pressing the button closes the circuit) since they are a lower resistance than the L2/R2 ones.
You may also want to install a couple of Resistors in there in parallel across the existing L2/R2 connections to lower it some since they aren't going to be as sensitive as before because of the resistance difference. They will still work without them though.
It can be done, and while it's not the easiest thing to do on a PS3 controller, there are much harder things to swap around.
This post has been edited by RDC: Nov 14 2011, 01:20 AM