The NVIDIA proprietary driver blacklists the nouveau driver by creating a file in /etc/modprobe.d
IIRC the file name is nouveau.conf, but I could be wrong there.
If you want to revert to using nouveau you will have to:
(i) remove that file
(ii) run sudo depmod -a
(iii) rebuild initramfs by running
sudo update-intramfs -u -k all
(iv) reboot
Hope this helps