Why would you need to reapply when you have an A already? If you didn't want to go to a DO school you shouldn't have applied to them and rescinding the offer could be problematic if you were to reapply. Also the likelihood of doing worse than a 512 is more likely than doing better unless you were scoring much higher on practice exams. Your MCAT is probably not the problem with your application. I doubt any school including UMiami will seriously consider a new MCAT in mid February when they are almost done with interviews. Just be patient and relax. You have already had a better cycle than most applicants.