Just another thought..... according to this
"When you open an older project in Logic Pro X it does not change the original project in any way. Instead, a new copy in the Logic Pro X project format is created which leaves the original project in place. You can continue to work with the original project in the version of Logic it was last saved in if you want."
Does this mean the 9 project will open with the changes made in X, if it's just adding audio files to the arrange?