Switching....
Although it's not impossible, it is frowned upon. Why? Because in awarding the scholarships the US Navy has run their numbers and they are anticipating commissioning "X" officers 4 years from now - less an average attrition number. Likewise, the USMC has done their manpower numbers too.
Also, if you're a Tier 1 major the Navy is going to strongly try to keep you as they need engineering officers to run ships. The USMC doesn't need engineers to lead platoons and companies though.
Good luck,
Bob McMahon