We'll try to keep your TUI boot working while testing just how to do this with the GUI boot, but from here on, you should be prepared to boot from a different medium (install disc should work) and mount the installation in order to make changes allowing you to boot at all.
As root, copy the present /boot/grub/grub.cfg to a safe backup location. Then edit the linux16 line in the existing /boot/grub/grub.cfg:
linux16 /boot/vmlinuz-3.18.16 root=/dev/sda1 ro quiet splash vga=788 forcepae
This may work. Even if it does, there are other changes that need to be made in order to keep it maintainable, so please post back with your results. It's also possible that there's something in the initrd that won't allow it, though I have no firm reason to suspect that. If there is, that can be worked around too, but it's much more detailed.
Just making the above edit and rebooting skips at least a few in-between test steps, but it's worth a try since there's nothing useful to be learned by doing the whole process if this does work.
If this does not work, please let us know whether having a graphical boot splash screen is important, or is it OK to have boot text scrolling by, then the regular graphical login you're used to?