I've read a few things but can't get a definitive answer.
So to confirm, is it
sudo ubuntu-drivers devices
sudo apt-get install nvidia-381
and then to switch back, it would be simply
sudo apt-get install xserver-xorg-video-nouveau
So just to clarify, to switch to the video drivers you want, you're essentially installing them rather than just enabling them?
Yes
sudo apt-get install nvidia-340
would be how you switch to the Nvidia-340 driver. Basically you're installing the drivers like software and the old driver is automatically uninstalled.