If you think it’s easy to change majors after transfer you may be wrong. For the UC system, you will need to declare a major. Each campus 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 some campuses. 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.
Private, out-of-state, and international colleges and universities vary in their policies. Consult with each.