The way I determine if I have too much flow, is if the fish are pinned up against the tank's sides or can't swim. Now if you have corals, that could be a different story as some like more flow and others like less.
Switching from CC to sand. I have done this. I like to be cautious and when I did it, I only removed 1/4 at a time, per week. I feel this gave the beneficial bacteria and sand critters time to migrate to the new substrate. Also, you will want to have plenty of PWC
available in case you get a swing in your parameters.
Note: don't use a glass cup or container to scoop up the CC (I found out the hard way and there is still a small pieve of glass under my sand somewhere)....