Yepp greylady is correct :) the majority of beneficial bacteria lives in the filter media...and I'm sure you know to not change or replace your filter media unless it's falling apart
a little bacteria lives in the substrate, but not a significant amount to completely mess up your cycle....and even if you go through a MINI cycle, don't worry about it too much...since there's only one fish in the tank, a little bump in the cycle won't be too bad...just test your water a few times just to make sure...I'm sure you do enough water changes to not worry aout it anyway
To change it, maybe siphon out half the water (to put back in the tank when you're done)...then siphon out all the icky stuff...you could always do a 100% change, too...as already stated (by myself and others), most of the bacteria lives in the filter media, so you'll be fine.
Post pictures when you are done! I love the look of sand! I'd take it over gravel any day! (and my some of my bettas used to like to lay down on it
Edit: forgot to mention, don't rinse out your filter media! And keep it in old tank water while you're doing the swap! And try not to let it set too long in the stagnant water Not completely sure, but I think the bacteria needs aeration? Just a little rumor I heard a while back