CADKEY .cdk files are not backward compatible and cannot imported to a previous version of CADKEY to produce the previous version saved as .prt file. The only option would be to use some sort of exchange format like IGES to STP as a middle step, but not all CADKEY features may be preserved that way.
This conversion entry was verified by us and conversion was flagged as wrong or not possible.
Usually this happens when the conversion would try to convert different types of data between each other, such as audio to image, or it is not possible because incompatibility between programs, caused by closed file formats, very limited support etc.
If you find entry that you feel is flagged as bad wrongly, please tell us in the discussion below with as much as information as possible so we can fix it.