When people imagine the future, they do so by projecting who they are today into that future.
Similarly, when they remember the past, they do so by projecting who they are today into the past. When they are remembering the past, their current self causes them to remember the past differently.
As a business analyst, there are many times when the client has come to a new understanding but forgets what it was like before they had the understanding. They bring their new understanding back to the past.
For consulting engagement, this is an important concept to understand. For situations where you want the client to understand how far they have come, you cannot rely on their memory. That is why documenting the current state is so important.
It is the same for getting testimonials. The best time to get a testimonial is during the process as they are having different transformational experiences. If you are getting the testimonial as the process is going on, the client is likely to understand how big the change was. If you wait until the end of an engagement, the new state may be the norm, so the change is not as noticeable.
Most importantly, always have things documented.