In my experience, the chick starter seems to be a good basic food for the first 4 months. Plan to stick with that for now. My babies have never been real fond of fruits and veggies until they were a bit older. They didn't know what to do with worms either, however, at about 2-3 weeks old, if you buy crickets at the pet store, the chicks go nuts for those!! And its hilarious to watch them chase them around the brooder. The small ones are the perfect bite size for chicks!
After about 4 months, their palates get a bit more adventurous, and they enjoy fruits, real worms, yogurt, oatmeal, etc. See the "chicken treats list" here on BYC for a great list of treats for older chickens. I keep crumble in their coops. That is my chickens' primary food. Almost every morning, I'll give them a little treat like fruit, raisins, or yogurt. I save the scratch for "extra" when they are free ranging. They love the stuff, but I know its not as nutritionally complete as their other food.
As long as the food stays dry, I wouldn't worry about changing it more than once a week or so. If it gets wet, change immediately, as sour food is bad news. (Edit: just saw your mention of mixing with water... as long as its eaten up right away, I think you're okay)
Have a great time with those chicks! And congrats on your chicken adventures!