Jonah Lehrer describes how memories are created, remembered and then recreated in The Forgetting Pill in the March 2012 issue of Wired Magazine. Apparently, new studies on how memories are remembered show that memories are reformed each time they are remembered. They may be reformed unmodified or in some cases, they may be changed. This process means that traumatic memories may be modified so that they are less traumatic. It also mean that our memories are susceptible to incorrectly remembering events, or even to remembering things that never happened.
The simplest way around changing memories is to preserve them, preferably in Creative Memories photo books. After all, unlike our memories, high-quality photo books do not change over time.