Drop out of PhD to do MSc
People change programs, this happens, and your advisor may or may not be understanding and supportive depending on the circumstances of your case.

Don’t misrepresent yourself in your CV! If you were a graduate student, say that, and mention that you switched programs in your statement of purpose (saying you did so due to personal circumstances is fine). If you say you were a research assistant and mention nothing about the PhD it’ll look like you have something to hide.

Good luck!