I would absolutely, 100%, without a doubt, alert the schools you have already applied to that you plan on retaking the test. Otherwise, they will make their decision solely on the information they have, i.e, your previous score.
I, for one, am not a fan of applying after taking the February test for a variety of reasons. Primarily, because even if you score on the higher end - you do not get your score until some time in march and most schools have already sent out most of their offers by then and their incoming class is pretty much filled out. Think about it, while you are waiting for your new score, most people already know where they are going.
but, if you plan on taking the test, without a doubt, alert the schools you have already applied to. Send a letter, or an email, or even call them.
-Peter