If you're living and working in California, there's not a whole lot of difference in taxation between a part-year resident and a non-resident who happens to be working in the state often; it's more of a problem when California considers you a full year resident and you're actually working somewhere else. For residents, during the period of residency, all income is treated as California source income, but for non-residents, California only taxes income that is actually from a California source (basically earned income from working in California, or gains on property in California).

Even if you're considered a resident during YC, you wouldn't be taxed on non-California source income before you moved in, or after you moved out. It's usually not a problem when you legitimately move; it's more of a problem when you keep a house in California, and visit frequently, and still get your hair cut in California, still vote in California, etc... Or in the case that you move overseas --- there's a presumption for US citizens that an overseas move is not a permanent move, and that when you come back, you'll return to the last state that you resided in, and many states with all-source taxation for residents will make a strong suggestion that you're still a resident, until you establish residency in another state.