Running ubuntu 18.04 with nvidia-driver-390. How can I switch to nouveau and make the change persistent between reboots?
You can switch drivers by running the GUI Additional Drivers from the Software and Update app.
Or you can do it from the terminal commandline by running these commands:
$ ubuntu-drivers devices
This will show you your available drivers. Then pick the nouveau driver found and switch to it with this command:
$ sudo apt install [xserver-xorg-video-nouveau]
[xserver-xorg-video-nouveau] is an example a nouveau driver that might be found.
If for some reason you have removed your nouveau drivers, you can reinstall them with (for example):
$ sudo apt install nouveau-firmware
You can verify the available nouveau drivers from the repository with this command:
$ apt search nouveau