Does it matter if the Stick has an old one piece truss rod or the new twin rods? I believe it does, otherwise the change would be unnecessary. But - would you consider buying a second hand well cared for Stick with the old single rod a good affair?

Sending a Stick from Sweden to California and back for a retrofit would be an economical disaster. Anyone out there with a single rod Stick who is happy with it? Yell YES! Otherwise just tell me why. My options aren't that... optional over here.

I have to ask...

Dual truss is a relatively new enhancement and while an improvement, it is not a necessity. There are lots and lots of Sticks out there with the single adjustable truss rod and they are fine instruments and easy to adjust and maintain. Given the option, I would go dual truss but If I were looking at used instruments, single vs dual truss would not impact my decision and if I bought the single truss, I would not send it for a retrofit.

I have both. One stick with a single truss rod, and another, nearly identical stick with a double truss rod. I love them both, and they both have excellent action. Both are easy to adjust, and both play very well. I would not hesitate to buy a single truss rod stick.

I thought it would be that way...
Anyway, I had to ask the question.
If I don't remember wrong, my old ironwood with number 128X had two non adjustable rods "epoxied" into channels at the back. I also had a Steinberger bass that was non adjustable, so I never doubted the stability in the old Stick's construction...

I have owned both an ironwood and a poly carbonate Stick and both could have benefited from having adjustable truss rod in them. For anyone who has one or is buying one without an adjustable rod (not previously retrofited), I strongly recommend that they send it to SE to have one installed.

I would agree. But just to be clear ... Russell is talking about much older models where the truss wasn't adjustable at all. I think all of the instruments built since ... I want to say 1990 ... have had adjustable truss rods.

I have an Ironwood from 1983, and it had been retrofitted with a single adjustable truss rod. It's perfectly fine - it works great and I don't find myself wishing I had the newer truss.

Based on what others say, it seems the 4-way truss rod is more of a luxury than a a necessity. Although it's very nice to be able to adjust both halves of the board and I consider it a great breakthrough in stringed instrument technology.

Others have said that they can't understand why guitar makers don't license Emmett's truss system for use on their own instruments. Who knows? It could boil down to simply aesthetics, and the lack of desire from guitar and bass makers to introduce a new look on an extremely established instrument form.
Drums went through this with suspension mounts. The hardware that mounted toms to stands (or bass drum rails) was always bolted right to the shell - a total resonance killer, but the look was very clean. Along came Gary Gauger in the very early 80's with his RIMS mounts (Resonance Isolation Mounting System), and he did well with it as an aftermarket item, but drum manufacturers didn't license it, and they didn't come up with their own versions of suspension mounts until a good 10 years later.
Thinking out loud, I also wonder whether using Emmett's truss would result in the removal of too much wood for a small neck to handle? But then, a smaller neck and instrument should be able to get away with a smaller rod, so the only issue you're left with is the look of an exposed rod in the back of the neck.
Well, also the issue of the player's hands coming into contact with it. A guitarist's and bassist's hands wrap around the neck entirely. We Stick players use two hands and our thumbs are the only part of our hands that even get close to (let alone contact) the truss.
Nah, it's probably an inability to overcome a conservative mindset, and aesthetics.