That is what I just did. Phone is rebooting now...aaaaaand I think it f'd up my ROM...

edit

ok, didnt mess up the ROM, but with GTX, installing CHT over the top, the CHT graphics do not retain the GTX theme, it installs the default CHT ones, obviously.

so, that being said, hold off on CHT 1.8 if you are running the GTX ROM.