As a newly forming memory develops (see chapter on memory consolidation in my book), it is susceptible to disruption by mind wandering, other stimuli, distractions, etc. When a new memory is retrieved, a re-consolidation process will be required if updated information needs to be incorporated. Such re-consolidation involves a new round of protein synthesis in brain cells, similar to that which is needed to make the initial learning a lasting memory. Likewise, a re-consolidation process must be protected from disruptive influences if the updated information is to be integrated and consolidated with the original learning.
There is good news and bad news here. The good news is that new information can update and be integrated with old memories. Recall also my earlier post on the possible use of this principle with treatment of Post-traumatic Stress Syndrome. The bad news is that old memories become vulnerable to corruption with each new re-consolidation, leading perhaps to false memories. My book has a whole chapter on false memory.
Source: Rodriguez-Ortiz, C. J. et al. 2005. Spatial memory undergoes post-retrieval consolidation only if updating information is acquired. Soc. Neuroscience Abstract 654.20.