Here is a quick how to. This is for the EAA but pretty similar. I have done this on mine eventhough I am a right hander.
One note: I wrapped a strip of electrical tape around the Grip in order to hold the mag release spring from jumping out.
http://pistolsmith.com/cz-pistols/13271-how-change-mag-release-eaa-witness.htmlField strip the gun.
Inside the frame, under the trigger bar, just behind the trigger is a spring loaded plunger. This is the trigger bar spring assembly.
Press the plunger down and rotate it to the rear.
Pull the plunger up and out of the gun.
WARNING: The mag release spring and plunger will pop out, don't lose them. These are inside the hole visible on the side of the release.
Reverse the mag release button.
Make sure the spring and plunger are in the hole.
Using three hands hold the mag release in place, insert the trigger bar plunger assembly into the hole behind the trigger. Use a tiny screwdriver to press in on the mag release plunger, and press the trigger bar plunger assembly down until it locks in place.
Rotate the trigger bar plunger around to the front to it's seat under the trigger bar.
The only problem is pressing the mag release plunger into the hole in the release, while also pressing the trigger bar plunger down from inside the frame.