FD10 opens FD9 / 8 files directly. You should be able to simply double-click on a script file icon and it’ll open in FD10, if that is the most recently installed version of FD on the computer.
There is no need to copy and paste any content from a script into a blank FD10 document.
If you have FD10 installed and when you double-click on an FDX script icon it opens in FD9 / 8, the file association needs to be reset:
If you have Windows: http://www.thewindowsclub.com/change-file-associations-windows
If you have Mac: http://www.laptopmag.com/articles/how-to-change-default-applications-mac
You can also open FD10, go to File > Open and open older scripts that way.