For the UC system you will need to declare a major. Each campus and each major has its own policy regarding changing majors so you will need to check with a university representative.
For impacted programs, most campuses will not let you switch to the impacted program after transferring. For the CSU system you may transfer without declaring a major. To be admitted to impacted programs, however, you must apply to the program. The CSU system strongly encourages students to declare a major by the time they have 75 (semester) units earned.