A bit later, I had the stuff I needed. I disassembled the busted case lock, first. That left the good one to use as a model for reassembly. It doesn't take much to get the lock out of the case, something to consider when you imagine a plastic case offers some kind of security. They don't. I accidentally "locked" the broken case, thinking I was going to set the mechanism free. All it took was a flat screwdriver and some wrist torque and the lock popped open; just like it would if a thief decided to empty my luggage on the road.
Once I had the case off of the bike, the first step is to remove the lock screw. This one screw holds the entire latch and lock mechanism in place.: one #8 by 1/2" Phillips screw between the world and all of my stuff. GIVI, security is not thy name. The screw out, the lock comes loose and you're ready to disassemble.
Dis-assembly is easy, too. One e-clip holds the lock and latch in place. Pop that with a flat screwdriver and the latch comes apart easily.
Once you have the latch apart, you're ready to reassemble with the new pieces.The new key lock goes into a plastic surround.
The surround pushes into the latch assembly.
The latch assembly is held together with the e-clip. That is most easily re-installed with a pair of needle nose pliers. 
Once the clip is in place, slide the latch mechanism back on the case.
Tighten the Phillips screw to hold it all together. You're done.
In my case, do it two more times so that all three of my GIVI cases use the same key and I'm set to go another 50,000 miles with this luggage. The fact is, it's probably a good thing I didn't think about how fragile my "security" has been with these cases and it's a good thing that I know that now. I'll probably buy some bicycle cables to lock around the cases, when I'm away from them from here out.
