Is it possible to change the retention period of filed Documents?
What if you need dynamic retention periods? One possibility is to create 2 storage areas and move the files from storage to storage. But this is not a good solution. Is there any other possibility to solve this problem?=
Storage areas only address the problem of storing the "content element" (files). Yes I suppose that, if the underlying storage devices have some sort of disposition mechanism, this might allow you some control. But it won't remove the associated metadata, and it frankly does not look very clean... and you will need some custom mechanism to "move the files".
The Records Manager product is designed to deal with the problem of retention and disposition. Since your question is rather broad, I will simply encourage you to read the "RedBook" on this topic: http://www.redbooks.ibm.com/abstracts/sg247623.html.
If you do not have Records Manager, I would encourage you to implement disposition via a custom component that deletes the content using the CE APIs rather than at the storage level. This will "cleanly" get rid of everything and you will be able to use the document metadata to better control the disposition characteristics.
