There is a database diagram here: http://ez.no/products/ez_publish_cms/documentation/reference/database_diagram
There is also a heap of technical information on http://pubsvn.ez.no/ specifically http://pubsvn.ez.no/doxygen/hierarchy.html
In terms of content migration, eZ publish doesn't handle it as part of the core. There are however several addon contributions that do handle data import/export http://ez.no/community/contribs/import_export
