Yes, some of them, but it's only the minority. Most of Knetz are so happy about KYJ's casting news. I also checked Park Bo Gum's DC when the news came out and all of them are so positive and glad about it. Meanwhile some Inetzs are strongly against it.
I think knetz are more familiar with the original novel and yet they agree about this casting because they know that this drama will not 100% follow the illustrations and skinship in the original story and they will tone it down since it's a youthful romance drama, sageuk, and moreover airing in KBS which is a broadcast network that more conservative compare to the other networks.