If you reschedule now, there's no penalty, and no note will even appear on your record that you originally signed up for the exam.
If you take the LSAT, you have six days after the test to cancel, and a you won't get a chance to see your scores before you cancel (or after, for that matter). A note will also appear on your permanent record indicating you cancelled, and that test will count as one of your "three LSATs within two years" limitation. So, if you aren't ready, don't go in and try it just to see what it is like--there are too many downsides.
I couldn't agree more. At this point, if you know that you'll need more than two months' worth of intense studying to even come close to the score that you're looking for, it's not worth it to go in and take the test "just to have an actual testing experience." If you, for whatever reason, miss the cancellation deadline, you'll end up with a potentially bad score on record that will then make the potentially higher score you get later (after tons of studying) look less impressive. If you do end up cancelling it, you'll end up not only taking up one of your chances to take the LSAT, you'll also end up with a big fat "CANCELLED" on your LSAT score report (and for no real reason).
IMHO, it's better to just reschedule it for October now, if you're already considering cancelling it due to lack of preparation. The deadline to change your test date is May 13th, so you still have time to decide if rescheduling is the right thing to do but, just judging from where you're coming from, it may be to your benefit to just give yourself the summer to study calmly and really get into the LSAT.