My solution: I carefully clamped the threaded fitting on my old thermocouple end-to-end in my vice so one face was up, then used a 1" cutting disc in my dremel tool to cut a slot into the fitting so I could slide it off of the tube. I slid the threaded fitting on my new thermocouple down out of the way, then slipped the slotted one over the tube and carefully screwed it into the valve fitting. It works perfectly.
If you have a several years old Whirlpool or American Products water heater, you might want to check to see if it has the left hand thread on the thermocouple. If it does, and it is still in warrantee, contact the manufacturer now so you can get the conversion kit for free ahead of the time you really need it.
The early symptom of failure on my thermocouple is that it took 1 1/2-2 minutes of holding down the pilot light to get it to re-light and stay lit. The new one took about 20 seconds.