My System76 laptop, pre-installed with customized Ubuntu, needs to be rebooted just to switch between Nvidia and Intel GPU if you want power savings (external monitors are directly wired to Nvidia chip).
That's an ubuntu 18.04 thing that's being worked on. You can either stick with 16.04, or use this repo https://github.com/matthieugras/Prime-Ubuntu-18.04 until it is properly fixed. More importantly, the first rule of running linux well (which I assume you wanted to since you got system76) is to never buy nvidia.