Hi Mark.
Note that the 5 W 5.7V Zener Diode is physically quite a bit larger than the stock Zener found on the Rev 1 kit SMPS 5V SMPS board. The 5W Zener won't fit directly onto the board, so it has to ride atop a couple of insulated wires in order to fit.
I've done this mod successfully on my Rev 1 QMX kit 5V SMPS board, but it's not exactly easy. I soldered 2 insulated leads to the 5W Zener, then after removing the stock 500mW Zener, I was able to solder those 2 leads to the pads on the board. There's no room to do that with short wires, so I made them a tad longer so I could reach in there with a soldering tip. Afterwards I was able to grab the 5W Zener with a tweezer and twist it 180 degrees so that it pulled itself down nicely between the other nearby components. Then I glued it in place with electronics grade silicon. Overall, not an easy task.
If you feel compelled to upgrade, which is entirely optional (unless your Zener has already burned out), then QRP-Labs offers a pair of replacement SMPS boards for the QMX which already has the 5W Zener pre-installed on the 5V SMPS board (note: a larger Zener is not however, recommended for the 3.3V SMPS board).
After doing that mod once myself, I'd go with just getting the very reasonably priced SMPS replacement set for the QMX from QRP-Labs instead of repairing the board. (The replacement boards were not yet available when I did my modification.) Or just go ahead and do the repair with the 500mW Zener you've already have on hand to get you back on the air (if that's the case) ... and perhaps order a new set of SMPS boards as a backup.
--
73
de Roy - KI0ER
Littleton, Colorado USA
Mark K9TR