A change in insulin because of insurance

My insurance company wrote me a letter recently saying they no longer covered NovaLog Mix 70/30 and I had x number of days to change my prescriptpion!!! What is this about?

I've used NovaLog Mix ever since I went to insulin about nine years ago. One of the doctors I use said I would probably be put on Humalog. No problem. I hope not.

Anyone have problems changing insulin?