This is a somewhat mundane technical detail about MCATs and the AMCAS application but we've gotten quite a few questions about this this year.
A number of applicants each year decide to retake the MCATs. Assuming this happens in the summer after the AMCAS application has come out (early June), the question is how to notify medical schools of their upcoming test date and new scores.
The main thing to know is that the AMCAS verification process runs completely independently of MCATs. For instance, if you do not yet have an MCAT score on the books, this won't slow don't the verification process. So long as you are committed to applying to medical school no matter what score they receive, you should submit your AMCAS as early as possible and not wait for your score. In the case that an MCAT score comes out after your AMCAS has been verified, you do not need to contact each medical school about your new score (so long as you've indicated in AMCAS this new test date). Schools are automatically notified.
Here is what the AAMC says about the process:
Applicants can, at anytime pre or post submission, let medical schools know they plan to take another MCAT and the date they plan to take the test. They log into their AMCAS application go to the STANDARDIZED TEST tab (Section 9) and select Next MCAT from the options available on the right, they’ll be presented with the list of MCAT dates and they need to select the date they plan on taking the test. If they decide, later, not to take the test, they can change their answer again. They need to be sure to re-submit their application by selecting SUBMIT APPLICATION on the Main Menu in order for the changes to save and be sent to the medical schools. This will have no affect on their processing time; if they are already processed we will make the new data available to the schools.
MCAT scores are automatically inserted into the AMCAS application, if the examinee has applied to AMCAS, once the scores are available. The applicant doesn't need to take action once the scores are in, he or she can check the AMCAS application though to double check that the scores appear in the application.