Here is a thread from the last admissions cycle, with two more threads inside it: http://talk.collegeconfidential.com/prep-school-admissions/1845657-communicating-with-ones-top-choice-school-s-p1.html
We struggled with this ourselves. As you will see on these threads, the opinions on sending a first-choice letter are split. You need to base the decision on your specific situation. We needed lots of FA and were reluctant to commit to one school before seeing all offers. And, as you noted, impressions and opinions change after M10. I personally would keep all options open.