God I hate Xf86config and xorg conf files. And we're flush with virtual servers these days. This occurred on a headless physical, but the steps should be the same to change from a forced 1024x768 to 1920x1080:
# init 3 to kill your window manager.
# X -configure to create an xorg.conf.new
# cp xorg.conf.new xorg.conf to overwrite the old with the new.
# init 5 to restart your graphical environment.
I use NoMachine (nx) which has a Display --> Change Settings --> Custom Resolution functionality I used to great success after the above steps.