Half step down shouldn't be an issue - you'll probably need to tweak the trem setup a little, but just a small adjustment to the claw screws, I'd think. Take a picture, make a note of where the trem is now, tune down, then adjust the screws evenly. Going slow is your friend.
Dropping the low E to C# would likely involve a little more adjustment to the trem claw screws, and maybe a truss rod tweak.