None select a major until mid-semester, plebe season. So ...major (except in some language areas where plebe start might be essential) plays virtually no role until youngster year, for most Mids.
And ...generally speaking, beyond a course here and there, and some freed up spots due to validation and/or summer school, relative to most secular institutions, not a great deal of flex in the system.
In fact, each year, USNA loses a student or several because they are serious students of oceanography, architecture, etc. and want more than they can get at USNA. No matter the major, USNA is a "whitman sampler" type curriculum not much overdosing in any specific area. Arguably, a USNA ME degree is not comparable to an MIT, Cornell ME degree. Thankfully. Equally arguably, a USNA international relations major may be immeasurably superior to a Gtown certificate of the same ilk.
And understandably, as virtually no newly minted Ensigns and 2nd LTs are put into major-specific roles. USN is big in OTJT and eventual post-grad specialization education for those who choose to stick.