What is it that you do not like about your current job? Not to discourage you, but you should read about some disgruntled nurses' perspectives on here before making a career change, and talk to some actual nurses first. Since you're making a career change, you want to ensure this is the career you want. There are many great things about nursing, and I enjoy being a nurse, and do not regret the decision I've made, but I do wish the hours were better. I don't mind working hard, but working night shift is very difficult on a social life and quality sleep. I do not get quality sleep, and many night shifters don't, but I make the best of it.
You will work night shift when you graduate. Maybe you won't, but odds are stacked against you since it's only fair that those with seniority get day shift.
Also, I went into nursing for the same reasons you described, volunteering and medical missions. I have 2 years of experience now, so I feel ready to do so. However, financially, I can't afford the trips overseas.....they're astronomical for a short period of time, so I'm going to adapt my plan somewhat. There are needy people in the US, too.
I will also say: I also wanted to volunteer at the free clinic, but to be honest, working long nights as an ICU nurse, I decided that my free time volunteering needs to be away from nursing. It's just too much otherwise. Maybe you'll feel differently, though.