I recently got a 2016 Ford Focus Electric. Before that, I was driving a 1996 Toyota Camry Wagon. I've been told many times that one needs to come to a complete stop before changing gears or else risk damaging the transmission. However, electric cars have a one gear transmission, so I wasn't sure if this same concern applied to them. Broadly, do I need to come to a complete stop in an electric car before changing gears? Specifically, do I need to come to a complete stop in an electric car before switching to low gear from drive, or to drive from low gear?
The Ford Focus Electric doesn't have a transmission per se, the gear selector is actually a mode selector for the electric drivetrain. Switching from D to R changes the polarity of the electric motor, causing it to spin in the other direction. When you are in Drive the car is in "Auto Regeneration" mode, meaning that whenever you lift your foot off the pedal it will automatically slow you down by using regenerative braking (changes the motor into a generator and converts kinetic rolling energy back into electric power). Low mode engages more auto-regeneration, slowing you down much faster if you lift off the pedal, possibly useful if you want to go down a hill and don't want to ride the brake (which also engages regenerative braking). Low mode has no benefit for going up a hill as it doesn't change any sort of ratio.
So, changing from D to L can be done at any speed without any sort of issues.
There's a discussion of the benefits/drawbacks of D,L and N modes here.
All automatics(about 1998 and newer) I have driven just won't shift to the gear you desire, if that would result in the engine revving either too high or too low. So for instance shifting to 2nd while driving 120km/h would result in the gearbox shifting to 3rd gear, since that's the lowest it can go without overrevving the engine. Same for choosing high gears while going slow, it'll just shift to the nearest gear that is still within safe operating area of the engine. So it's designed pretty foolproof. I don't know if this holds true for all automatics, I suppose it can be different with older ones.
It's probably different with vehicles than can select 4x4 or a higher or lower gearing all together. Newer automatic electric vehicles like the Tesla will simply respond with a beep or something if you try to select Reverse while still driving forward.
I guess the same is true for your Ford. I have never seen or heard from an automatic electric vehicle that lets you directly, mechanically control the gears. In all cases I have seen, it first checks if it's safe to fullfill the drivers demand before it will actually do so.
NB: this is my experience with newer vehicles which are always stuffed with electronics to check for safety. Older automatics from the former century may act on the drivers demand without checking for safety.