Change of Major

Hello, I am a senior at a California high school and new to this forum. I received a general letter from Boston University based on my information sent by College Board website. I chose Aerospace Engineering (AE) as my possible major in the website. I am also interested in Mechanical Engineering (ME) and am confused which one to take. Since the letter mentions invite to pursue AE, do I have to chose it or can I change the option to ME? Will I lose the chances of getting in if I chose ME? If so, can I change my major after accepting (if I get in). Please bear with me as I have too many questions and very little help. TIA