Welcome to the forums. Now to answer your questions:
It's hard to start a career in software engineering because most companies require at least one year of experience. Just go to careerbuilder.com and search for "programmer" and you'll see what i mean. So, if you decide to become a software engineer, try your best to get an internship or coop before you graduate. This will increase your chances of getting a job later.
I'm not sure what you're asking in your second question, are we talking about a last minute major change? If so, what's your current major? if it is something close to software engineering, then it might be ok. But in general, it's a bad idea to change your major in the last year. However, if you're talking about last year of high school, it is ok to start with software engineering because you'll learn almost everything from scratch. All you need to know is how to operate a computer
