Can't say I'm more than passingly surprised. It always felt like RJ decided to make that change based on fan response. He clearly thought he had a good "surprise" in store for readers with Taimandred, but just as clearly the fans found it very obvious from the early recognition. I remember very clearly thinking Taim = Demandred right from his first scene and, until much later, after RJ said this wasn't the case, just assumed that Demandred has killed Asmodean.

In a way I think this is one of the places where the series went a bit astray. This is a much deeper change to the story than it seems on the surface given that RJ essentially had to create an entire new story line for Demandred and still carry Taim's storyline out. Given how little relevance Demandred ended up having I think he would have been better off just going with his original plot.