Getting switched from 28L to 28R, and back again in SFO is a normal occurance, especially since the runways are so close together. There are other airports where you can be switched to the other side of the airport at the last minute and this can be problematic for FMS RNAV aircraft, as a new arrival, transition and proper approach has to be loaded and briefed.
In fact, due to 28L and 28R non-standard runway separation in SFO, flights must be staggered as to not pass the aircraft on the parellel approach, and if you do, a special break out procedure is initiated for that aircraft to go around.
I always brief the passengers about this as it can be quite alarming to some to see another airliner flying right next to you.