Individual health insurance is subject to underwriting and is not guaranteed issue in California. The carriers may turn you down based on your health condition or health history and refer you to state sponsored plans.
Once you are offered a plan and enroll, you may not be canceled (except for non-payment of premium or fraud). However, if health conditions arise after the policy has been purchased, it is possible that you may not be able to upgrade to a richer plan or to switch to another plan from another insurer.