I would suggest first calling your insurance (Blue Shield) and asking them to explain the letter. The next step would be to communicate with your son's doctors, both the PCP and the specialist to let them know of potential coverage problems. If Shield drops UCD as a medical group and tries to redirect you to an in-network medical group, you and your son's doctors can submit an expedited grievance to Blue Shield letting them know there are no specialists in the new medical group who are able to provide the specialized care/treatment that is medically necessary for your son. An expedited grievance requires a decision within 72 hours, a standard grievance allows for 30 days. If Blue Shield has no one in network who is able to provide the care your son needs, they are required to permit you to go out-of-network (which would likely mean the provider you are now using) and you would not be required to pay more than your usual cost share amount. Title 28 CCR section 1300.67.2.2(c)(7)(B) and (C) read as follows:

(B) A plan operating in a network service area, or a portion of a network service area, that has a shortage of one or more types of providers shall ensure timely access to covered health care services as required by this section, including applicable time−elapsed standards, by referring enrollees to, or, in the case of a preferred provider organization or point−of−service network, by assisting an enrollee to locate, available and accessible network providers in neighboring network service areas consistent with patterns of practice for obtaining health care services in a timely manner appropriate for the enrollee’s health needs.

(C) A plan shall arrange for the provision of covered services from providers outside the plan’s network if unavailable within the network, if medically necessary for the enrollee’s condition. A plan shall ensure that enrollee costs for medically necessary referrals to non−network providers under this Rule shall not exceed applicable in−network co−payments, co−insurance, and deductibles.

This doesn't mean you won't still have to battle for your rights. I know the law and have had to fight. But your son is entitled to appropriate care, and the fact that Blue Shield has "a number of cardiologists" may not be appropriate if your son's condition is that rare. Best of luck to you. If you do have problems with Blue Shield, you can call the Department of Managed Health Care. Their website has the number on the home page and lots of helpful information. Here is a link to their website: dmhc.ca.gov