The best time to do it depend on which plan you have , and if yes or no you made extra usage !
But in the best case if you switch from higher plan to a lower one without extra usage it is better to change it immediately. If it is the opposite better to do it after 12th
Your best bet is to change it a few days before the renewal, so in your case around the 10th, Erin :) Yes you will be prorated for 1 or 2 days (probably no more than a dollar or two) but you'll know you are in time. Some wait till the last moment and risk being too late, which is much worse :)