When upgrading from 3.0 to 3.1RC1, importing the XWiki.WysiwygEditorConfig document fails

This happens because the menuBar property was changed from StringProperty to TextAreaProperty. Whenever object properties change their type, Hibernate complains about having duplicate entities with the same ID, and saving the document fails.

This problem was partially fixed for different number types (XWIKI-2214) and different list property settings (XWIKI-1621), but a generic solution which accepts conversion between any types of properties would be safer.