1. The answer depends on how well you did in school, TOFEL Test result,
financial situation ..... You could always change your major. The most
important thing is to be officially admitted to the University.
Take a look at this site and check how many different majors this
university has (e.g. of a university in the USA):
http://www.uga.umd.edu/admissions/academics/majors.asp#b
Besides Science and Business, there are many others you could
choose from. Don't worry, you will be just fine.
Depends on the university you choose, each major requires
certain credits (學分) from several fields. (you could use the
following link as reference for the requirements for econ major)
http://www.bsos.umd.edu/econ/info/ECONmajorcard071306.pdf
2. I don't think so. There are so many business related majors
that is not science related.
3. In fact, the numbers of credit you need to graduate with a
Business degree is much less than a Science degree. A lot of students
started with Science major, such as Engineering, and ended up
with a Business degree. That's because it is much harder to study
Engineering than Business. More credits also means longer time and
cost more.
Hope this will give you a better idea. Good luck!