Adding rock/sand to an established tank

[ManderTang]

I brought home a 14g cube with a mantis today. He certainly is a cute little guy! The tank has been up and running for about a year or so. I'd like to aquascape the cube a bit, maybe with more rock and eventually some mushrooms or hardy corals. My fiancee insists the mantis will "pop" more with white sand instead of black. I certainly agree, but how would I go about slowly transitioning to white sand? As far as adding rock, I could add a few pieces of dry rock, yes? This is my first marine and I'm a bit apprehensive about screwing things up.

Here are some pictures: