I know people cross-commission from the Corps into the Navy, so I imagine you could go the other way, too. Other people might have more data. If you're thinking of switching to ground, though, you'll have a tough time. There don't seem to be many slots open right now.
The best way to get accurate, current information would be to contact an OSO (Officer Selection Officer). Go to http://www.marines.com/ and fill out the form (it's under Resources on the College Student/Grad pages). That'll go to an OSO and they should contact you. Otherwise, look up your local office in the phone book.
