If the Ubuntu driver is installed already then you do not need any other driver - that is probably not causing the fallback mode issue - most likely an acpi issue that was triggered when you swapped the video card
1. Boot the computer.
2. Press & Hold the "Shift" key immediately after the BIOS screen finishes, until the Grub menu appears.
3. Select Advanced Options for LM, then press Enter.
4. Select "Recovery Mode", then press Enter.
4. Laptop boots to "Recovery Mode".
5. Edit Grub in Terminal: sudo gedit /etc/default/grub
6. Enable the following line (or add if not listed):
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash noacpi"
7. Disable all other "GRUB_CMDLINE_LINUX_DEFAULT="xxxxxx" lines by adding "#" and a space, before the lines.
8. Click "Save".
9. Update Grub in Terminal: sudo update-grub.
10. In Terminal: reboot
11. If Grub menu appears, select "normal boot".