I'm a PhD student in particle astrophysics (which I guess is half high energy and half astrophysics). I can answer questions if you'd like. I can also answer questions about switching majors in an Asian household...if you count American-born Indian as Asian. :)
Anyway, there's one thing I should warn you about. Astrophysicists don't do much in the way of space exploration. I've never had to do anything with NASA, nor have I built anything that went aboard the space shuttle. In fact, I've never done anything that's even remotely associated with manned space exploration. I guess the closest thing I'll do to space exploration stuff is analyze data from the Fermi Gamma Ray Satellite. Astrophysicists are physicists. We study science questions and probe space to learn more about fundamental physics. My area of research is especially physics-based, since I'm essentially a particle physicist with a telescope. I guess you could do observational astronomy if you want just map out the night sky. But if you're interested in boldly going where no one has gone before and all that stuff, I'd recommend working for NASA rather than becoming an astrophysicist. Almost all the people I know who work for NASA are mechanical engineers. It's not the same thing as civil, but probably closer than physics.
Still want to be an astrophysicist? If so, then I've got mostly good news for you: you don't need to change majors. You do, however, need to get a PhD, which is an investment of at least five years in graduate school. The bright side is that you get paid to go to grad school, so you don't need to go into any student debt. You won't need to take any undergrad courses in astronomy either (I never took a single astro course when I was doing my undergrad, and I'm doing well as a grad student). You will, however, need to have some basic coursework in physics. I don't know how much physics CivE majors do, but you'll likely need four semesters in freshman and sophomore physics, plus a couple of advanced undergrad courses (like mechanics and E&M). You can get accepted into most grad schools without having a complete undergrad physics education, though they'll usually make you take some senior undergrad physics during your first year of grad school (probably quantum and stat mech).
Now about employability. I'd wager to say that a PhD in astrophysics is somewhat more employable than a BS in civil engineering. The caveat is that you might not necessarily get a job that involves doing any physics. The last statistic I read said that about half of all physics grad students end up getting academic positions. A lot of high energy physicists and astrophysicists end up becoming programmers, software engineers, or (surprisingly) computational biologists. So majoring in astrophysics in grad school doesn't necessitate that you'll get paid to look through telescopes for the rest of your life. If all you're worried about is employability, the astro PhD is a good way to go. But if you're dead set on being a professional astrophysicist, it's really a roll of the dice.
Anyway, what I've said is based on my experiences and the research I've done into employment possibilities. So when you read this, remember that it's only one data point.
