I think it is actually possible with this novel to take a rather novel perspective and consider the ways in which Briony doesn't actually change that much as we see her in her three separate time periods. As a child she is shown to be deceitful. Although it is suggested that Briony as a child struggles to separate the real world from the imaginary worlds that she creates in her head, the guilt that she suffers after her testifying against Robbie indicates that at some level she consciously lied about what she saw. Also, if we look carefully at the story, we could argue that Briony is actually jealous of the way that Robbie pays more attention to her sister than he does to her, which presents us with another more disturbing motive for her actions.
However, if we have a look at the ending of this excellent novel, we could argue that the deceit of Briony as a child is something that Briony as an old woman continues to practise. She deceives us as readers by presenting us with a happy ending, while the real ending of the characters was anything but happy. Briony justifies this by saying that as the author she is able to decide on the kind of ending that she gives us in her novel. Whilst this is true, we could also view this as yet another example of Briony's deceit and her difficulty distinguishing between reality and fiction.