Mildly Infuriating

Power train conversion is reasonably simple. Just throw combustion engine and transmission box away, make brackets for electric motors and attach them directly to the wheels (with axles if necessary). Conversion of controls is (I assume) is also somewhat simple since existing brake system and power steering is quite straightforward to run with electric motors since you just need something which can run a belt drive and gas pedal is most likely already electric. For all the electronics you have plenty of space in where the engine used to be.

But. And there's a pretty big but. Batteries are pretty big and pretty heavy. On any given combustion engine car there's just no room for them (at least if you're after a conversion with similar range/power than a readily built electric car). And even if you cut the floor panel off and modify it to accomodate battery pack (or whatever the route you choose might be) it'll heavily affect weight distribution, frame stability and many other things, suspension included. Model S battery is apparently 540kg, so if you'll do a conversion to your corolla you might save around 150kg of weight by removing old engine+transmission but you'd still have additional 300kg of mass to deal with.

For a van which is designed to haul heavy loads from the start it might be pretty simple to just raise floor of the cargo space a bit but for a common sedan that's a whole another thing.

Swapping an engine is relatively easy if you know what you're doing.. If these kits can connect the electric motor to the existing drive train it wouldn't be too bad. Messing around with batteries big enough for an electric vehicle can be really dangerous though.