It works when I do it. For example, I can export a document to an XML file, manually change an item value, and reimport the file, updating the document with the new value.
When the error message says "the DXL and database are not replicas", it means that it thinks the replica IDs do not match.
If you think it might help, I will try to reproduce the problem by importing your DXL. Post it here, or send it to me using the address in my profile.