1) The new MCAT has a hell of a lot more biochem on it than the previous one and this biochem is not something you learn in introductory biology classes. Self-study would be difficult and why do even that given the other factors below?
2) As the poster above just said, you don't want to take the MCAT again. You want to take it when you're at your best and that's definitely not the case when you haven't taken biochem, which makes up about a quarter of the new exam.
3) You want to have your app submitted and verified by July so that you can get your secondaries done and have interviews lined up early. Many schools do rolling admissions and it is to your advantage to get your AMCAS submitted by late June, verified in July. Applying in the fall is like shooting yourself in the foot. Also, if you're applying after senior year, I would advise taking the MCAT in January of that year or May at the latest. If you take it in May, you can still submit your AMCAS but you won't know your MCAT scores before you apply, which is scary for some people.