Adopting a medically fragile child - ins. question

I have sooo many questions this morning! But I was wondering if anyone had advice on this one. We just found out that we've been selected for a toddler with a heart defect and other health issues. He's being treated at our local Children's Hospital, which is quite good. Of course, as a foster child, he is covered by Medical. (California's version of Medicare.) When we adopted our healthy son, we were told that we needed to immediately switch him to our insurance and Medical would be his secondary. Which wasn't a problem. But we have an HMO with their own hospitals and medical facilities, so I'm not sure he could keep going to the Children's Hospital. We don't want him to switch doctors when he's looking at major heart surgery within the next year. We also think he would probably get better treatment at a Children's Hospital. For those of you who have adopted medically fragile children or children with ongoing health issues from foster care, have you been compelled to change their insurance immediately? Are there circumstances when you are able to keep Medicare if the child is very sick? Because of the budget issues, I'm guessing they'll push us to switch him over right away, and I'd like to know what is possible.