There is very little bacteria in the substrate, so there's no reason to expect new tank syndrome after changing. As always with any major tank upheavals it's wise to test for ammonia and nitrite and feed sparingly (or not at all) for a couple of days before and after.
Rocks from the beach might be ok, but you need to know what they are and be able to soak them in fresh water, changing it every day, to get rid of any excess salt. I feel I ought to point out that taking rocks from beaches is illegal in many places, so do bear that in mind!