Psychologists have successfully modelled Long term memory as something they refer to as discrimination networks.
In addition to the parent/child links of the network, each node has an associated “image” (letter, word, sound, visual image, feeling, etc). Anyone familiar with mind maps will immediately recognize this structure.
Persisting chunks in long term memory can typically take of the order of seconds.
Merging new elements into the network can result in the weakening of the existing links causing memories to be forgotten. A relearning cost is then suffered to ensure both new and old are accomodated by the network.
It makes evolutionary sense to have a process that minimises the potential forgetfulness of extending our memory network. This would be a process that allowed our memory to be open for extension but closed for modification. Meyer's Open-Closed Principle is an echo of the minds process to minimise relearning costs.