I don't know (or don't remember) myself, but speaking for myself, when I first "met" James Hall, he seemed much more in the James Dean Rebel without a Cause model, minus the angst and plus a lot of sociopathy. His performance was twitchy and genuinely frightening. I was really creeped out by it. He was an excellent actor and [spoiler]I totally believed that his Willie (no pun intended) would have raped Carolyn or Vicki if he'd had half a chance and I totally get that Carolyn would feel threatened enough to pull a gun on him.[/spoiler]
I suspect because the character as played by Hall was so unsympathetic, that TPTB felt it he had already taken the role too far in that direction that he couldn't believably be pulled back enough to act as Barnabas' servant.
I, too, would be eager to know a more official reason why he was recast.