switching within L&S is doable. Actually you’re not even switching per se since yes, you are coming in as undeclared. You do have to declare no later than at the end of Fall semester, and in order to declare you do have to have the qualifying 3.3 for all 3 classes CS61A/CS61B/CS70 before you can declare.
Note that many CCCs offer a Data Structures class, so you may be able to take that as an articulation to CS61B. A couple offer an articulation to CS61A also. However, if you do an articulation, your 3.3 GPA qualification will then be based on 2 classes (or one class depending on how many of the 3 CS classes that you transfer over), which may be helpful or harmful. Do note that even if you do take a Data Structures class in CC, you can take CS61B over again.