SInce a lot of bacteria is alive in the gravel, you will lose some. If you want to switch substrates, I would suggest taking some of the old substrate and put it in some nylon stocking material, and put that on top of your new substrate. It will help the bacteria reestablish more quickly, and make it easier to remove later.
About pH. Since silica sand does not react with water, it will not affect your pH. If you used argonite (calcium carbonate-based sand,used in saltwater and some cichlid tanks,) then your pH would be buffered, and your pH would go up (AKA become more basic.) Hope this helps.