Hello.  Due to the complex legal issues raised, this response is to be considered a starting point and not an exhaustive response.  When a witness changes her/his statement more than once, oftentimes that would call into question the witness' credibility.  A general principle is that the closer in time to the event that the witness recalls the event, the more likely the statement of the witness is to be considered accurate.  However, in cases involving traumatic events, for instance, gaps or errors in memory may occur.  It is well known that crime victims, abuse victims, may experience issues in recollection of events. The matter certainly, is contested (challenged) in particular legal cases where credibility of a witness is in issue.  Here is a link to some information about witness memory issues: http://en.wikipedia.org/wiki/Memory_errorsAnother issue involved in certain legal cases, is that a witness may initially make a statement designed to 'cover up' the truth of an event: For instance, a person may provide a statement to police aimed at protecting her/his friend. The person may then have reason to change her/his statement, due to multiple considerations (including the person conferring about the matter privately with an attorney).Regardless, I recommend that anyone facing the issue of a witness changing her/his statement ought to confer privately with an attorney.  I recommend that in choosing an attorney for legal help and legal counsel and advice a person make several telephone calls, because it is critically important that you feel a sense of great trust and safety with the lawyer helping you.  All the best.