I had similar issue with my ThinkPad t510 that had Intel display driver and Nvidia GT218 [NVS 3100M} driver. This is how I was able to switch default display driver to NVIDIA.
First clean-up any third-party NVIDIA drivers (in case you have them)
sudo apt-get purge xserver-xorg-video-nouveau
Then perform the following steps (as listed above by Messie, except the last line which includes --reinstall to cleanup the existing installation)
sudo add-apt-repository ppa:ubuntu-x-swat/x-updates
sudo apt-get update
sudo apt-get upgrade
sudo apt-get --reinstall install nvidia-current nvidia-settings
Reboot laptop and hold F-1 to get to the BIOS setup (thinkpad BIOS Setup).
go to Display
switch "Graphics Device" from "NVIDIA Optimus" to "Discrete Graphics"
change "OS Detection for NVIDIA Optimus" to "Disabled"
Save using F10 and let the boot continue
Now your laptop should be using NVIDIA as you can confirm by
nvidia-settings
To revert just reverse the listed steps.