How is it possible that so many intelligence services were sure 12 hours ago that there was nothing untoward with this incident and now we have the exact opposite? Surely they are looking at the same data as before
First, these stories are always evolving as new information comes to light, more analysis occurs, and the proverbial blanks get filled in. It isn't surprising that thoughts and views on an event will change over time, even a brief period of time.
The other option is that Western intelligence services knew that the plane was shot down by a missile, but chose to not advertise that knowledge due to fears of compromising their intelligence gathering methods. For example, if they knew via a human asset, a spy, they wouldn't want to advertise their knowledge for fear of compromising said spy. They would wait for more information to percolate around the world and the news media when they could then claim it was a missile without fear of harming their source.
More than likely, it was a blend of both reasons, if not others, as well.