I only use open source drivers.

Install AMD driver (radeon)

sudo apt-get install xserver-xorg-video-radeon

Tell source to load readeo driver, create the file /etc/X11/xorg.conf.d/20-radeon.conf with the following content:

Section "Device"
    Identifier "Radeon"
    Driver "radeon"
    Option "TearFree" "on"
EndSection

Install Nvidia driver (nouveau)

sudo apt-get install xserver-xorg-video-nouveau

To tell Xorg to load nouveau, create the file /etc/X11/xorg.conf.d/20-nouveau.conf with the following content:

Section "Device"
    Identifier "Nvidia card"
    Driver "nouveau"
EndSection

To change VGA card

Uninstall the respective driver and install the other, the Xorg config file is the key here. Otherwise, Xorg cannot know which driver to load.