This article is from the FAQ, by with numerous contributions by others.
If you are changing programs (e.g. B.S. to M.S.), the student must apply for
a new I-20AB for the new program and submit to the foreign student advisor
within 15 days of beginning the new program. The foreign student advisor
will return pages 3 and 4 of the I-20AB to you and sends pages 1 and 2 to the
INS service center within 30 days. Your permission to work off campus is not
affected. If you are simply changing majors, but pursuing the same degree,
no notification is required.