I live in Palo Alto, therefore I will provide the information on Palo Alto Unified School District aka PAUSD. In theory, it is possible to transfer schools since there is a "process" defined in the district office to request transfer. However, many schools -- and particularly schools with highest GPA -- are experiencing an overflow in enrollment and as a result, had lotteries to draw *out* kids from their neighborhood school boundary to be placed into other PAUSD schools. As a result, there are no guarantees that you will get placement in the school of your choice any more.
It is true with a few Elementary schools and my neighbors with high school going kids tell me it's been the case with Gunn High for past few years. A tough fact of being a Palo Alto homeowner is that your child is not guaranteed a spot for being in your own neighborhood school, therefore I would not base my property purchase decision based on that assumption at all.
My close friend tried to get a transfer for her child last year -- for a legitimate reason for her and her kid. However, she was given a lot of "attitude" at the district office, and was treated quite condescendingly. She decided to drop the ball but that should illustrate the extent to which the administration can act rude in such circumstances.