Your declared major is more or less what you're locked into until at least some time during if not after 4/c year. I was a declared Mechanical, but switched to NavArch for a number of reasons in October of 4/c year. I believe they've since changed the system. Based on your talk of similar curriculum, I am guessing these could be the majors OP speaks of. If so, a change is virtually guaranteed, as are most either within Mac Hall (engineering) or from outside of Mac hall to Mac hall. Where people usually run into problems with change of major is from within Mac to outside of Mac.
In response to what Hollyrock said, I'm not going to say that nobody has ever been forced into a particular major... But I have never ever heard of it happening.