I would send the LOI first if you havent already, including all updates to that point. If anything comes up after that point, whether you started/finished volunteering, a class, or any other update, write another letter in letter format discussing what update you have since your last letter. There is no time limit between update letters, just keep sending them if you have something new to tell them. So keep sending 'em everytime you have a new update! As a last paragraph, always emphasize how much you hope to be accepted at that school, and maybe 1 or 2 reasons why you hope to go there. If its your #1 choice, I would always just write at the end that the school is in fact my #1 choice, and you are hoping for an acceptance to next Fall's class.
I wrote a lot of these during my grad program, I guess they worked, or at least helped out! Good luck!! 🙂