http://cogsci.berkeley.edu/transfer-students suggests that you will be expected to complete remaining cognitive science prerequisites and declare the cognitive science major in your first semester.
Probably means that you will have to do that first semester, noting that CS 61A and CS 70 overlap with L&S CS prerequisites. If you have already completed the equivalent of CS 61B prior to transfer, then you could also try to declare L&S CS if you meet the GPA requirements. Otherwise, declaring or changing to L&S CS may need to wait until later. See https://eecs.berkeley.edu/resources/undergrads/cs/transfer-prereqs .