For the UC system, you will need to declare a major before you transfer. Each campus and major has a different set of policies regarding changing your major, so you will need to check with a university representative.
When the number of applications received by a CSU campus from qualified applicants is greater than the number of available spaces, a campus is considered “impacted.” An impacted campus is allowed to use additional criteria to screen applications.
Campuses without the impaction designation at the program or campus level are open to all CSU eligible applicants.
Most campuses will not let you switch to the impacted program after transferring. For the CSU system, you may transfer without declaring a major.
The CSU system strongly encourages you to declare a major by the time you have earned 75-semester units.