The only thing that happens in car factories is the engine is bolted into the car. Engines aren't made in those factories, they're made in separate factories. Changing to BEV cars just means you ship in an electric motor and battery and bolt those into the body instead of an ICE. The only significant difference is having to deal with safety concerning the higher voltage of a battery pack, but that's not very hard (and the battery probably isn't charged before assembly anyway).