Hello again,
something even worse happens during the upgrade process: versions get deleted!
I set up a fresh alfresco, added 3 simple text files, made them versionable, made some versions (changing the content each time). Then I took the whole data over to alfresco 3.0, the migration finished without any errors in the log, however, only the last version of each item still exists - which is quite pointless, as they are equal to their corresponding current element, anyways.
I would consider both behaviors (this one and previous post) serious bugs. I have to trust alfresco, that I can preserve our data somehow and use it in recent versions. We are stuck to 2.9 for now.
Cheers,
Tim