I think my comment has been mirepresented. I want to prove my suggested configuration is "more legal" (at least under a reasonable assumption of the spirit of the law) than configurations with LCD allowing to set / modify max speed (by default at 25km/h):
I suggest starting with a "legal" motor, e.g. a 250W Bafang CST (or in Barcelona, say a 500W BPM under SEL legislation). Say I want to plug that motor into a KU123 controller, which is legal in relation to power, but very likely illegal in relation to speed: a 250W Bafang CST with a KU123 and a good battery most likely has a max speed above 25km/h.
So, now the biker has to do something. I suggested he bought a 3-speed switch, such that with the 1st speed, max speed becomes 25km/h, and he hides the switch inside a box (probably where the battery and controller are located).
Now let us assume two possible situations:
1. The one described above
2. A "legal" bike with LCD allowing to set / modify max speed (by default at 25km/h)
Which one is closer to the spirit of the law? In my opinion, clearly (1). The reason is:
a. A biker under (2) can easily change the settings while biking. Instead, a biker under (1) cannot do that, and probably he has to stop, get out of the bike, and click a button.
b. Biker (1) at least has done some effort to reduce his configuration to the legal limit, since he has spent money buying that 3-speed switch
Even if biker (1) made a hole allowing him to click a button, this would most likely be more difficult to do while biking than modifying an LCD set comfortable at the handlebar.
To sum up, I believe that adding a 3-speed switch to a KU123 (or similar) is as legal as (or even more legal) than having a LCD with the max speed set in there. In fact, having the 3-speed switch on the handlebar would be roughly equivalent to having the LCD, since in both cases, with a rapid hand movement one could change the settings.