Need letter wrote to insurance company to persuade keeping current

My son is autistic. He has had the same doctor for years now. I have used them for everything.. including getting the process started for his diagnosis of autism. He is now in therapy that is not being covered by his insurance. If we change our type of insurance, which would mean a change in doctors then his therapy would be free. I dont want to change his insurance. He does great at the doctors now. He used to be so scared. They give us extra time knowing he might throw a fit. I was told that I can submit a letter to the insurance angency asking to change insurance so therapy is covered, but remain with our current doctors.  

I can explain more later if anyone is interested in helping me out. 

Thank you