Computer Science is considered an impacted major so they take students on a competitive basis. If you wanted to switch to computer science, you would have to apply (http://www.cse.ucsd.edu/ImpactedMajor). You would, however, be able to take most computer science class if you wanted to, even if you are not a computer science major. These http://ucsd.edu/catalog/courses/CSE.html are the computer science classes offered by the department, and you can take any class that is not restricted to majors only (listed in the prerequisites), regardless of your major. The classes just wouldn’t fulfill requirements towards your degree (unless your major has a requirement that can be filled by a computer science class).