I considered doing this during my second year but keep this in mind: it is very difficult to transfer from a Cal State to a UC. The UC's reserve most of their transfer spots for community college students, many of whom are in guaranteed admittance programs. Also, transferring units can be tricky since the CSU's are on a semester system and most of the UC's are on a quarter system. You'd need to make sure that you complete course sequences (ie. Bio I & II, GChem I & II) before transferring or else you may need to retake courses.
I ultimately decided to stay at my CSU because transferring to a UC seemed to be more trouble than it was worth and the chances of admittance are very low for students coming from 4-year universities. For state funding reasons, they give priority to those who aren't already enrolled in a 4-year institution.
However, I know several CSU alumni who attended or are currently attending medical school at UC's including UCSF, UCSD and UCLA, so it is possible to get into one of these med schools from a CSU...you'll just have to do better on the MCAT and have a higher GPA to "prove" yourself since CSU's reputation generally puts you at somewhat of a disadvantage.