Provide migrations tool

when i have existing content and later change the document class to have referenceable=true, this is not updated even on persisting the document again.

uwe: I think there are several other things that fail when you have existing content in the repository - maybe the solution to think about is something like migrations ... that would prevent the performance penalty on ordinary code and still give you the change to change the structure of your documents.

things that come to mind

• change document class name
• removing fields
• change field type
• bulk changing the values in some field
• everything related to node types (though, as an additional challenge at least jackrabbit does not allow to remove things from node types through the davex remoting we use in jackalope)