A witness who makes a witness statement retracting is in for a tough time. They will be forced to attend. They must give evidence. When they do and if they give the retracting version, the original statement will be shown to them. They will be asked which statement contains the lies. Which one is the true one? If the later one, why did they make the first ? It will have been made soon after the event. Why would they lie to get a person who has done nothing arrested, charged, and put on trial?
You get the picture. There is a witness support service for anxious witnesses. It is useful.