This is the list of UCSD selective majors: Selective Majors . Basically, they are biology, data science, public health, and engineering majors (but some STEM majors like math and physics are not in that list). Other majors should not be difficult to change into if you complete their prerequisites.
Parent loans are generally not a good idea, since taking them implies that the parents do not really have the money (and if they did, they would have saved up the money beforehand).
Also, taking substantial loans when your career directions are likely to be low paid can mean a rather long term debt burden that you will not be able to help much paying off.