In order to go from older versions of FCP to FCPX, I would suggest flattening to a quicktime file, or just keep working in FCP until you’re done. It’s not going away. The old work around to go from old versions of FCP to newer used to be to export an XML File, but that looks to have been lost in FCPX. Maybe XML support will come in a later iteration. Also, for the first time, there is no backwards compatibility. This is because of the new Core Data architecture in FCPX. You can read about it at https://www.fcpxtutor.com.