If you've got no fish, you should be safe to change it all at once. Are you doing a fishless cycle?
Also I'm not sure if this is an issue or not, but that rock on the right looks like it could be limestone, which may raise your pH. If you want to know for sure, you can pour some vinegar on it. If it fizzes, it's limestone.
I should mention that I really like your layout and decor.