FD12 opens FD11 / 10 / 9 / 8 files directly. You should be able to simply double-click on a script file icon and it’ll open in FD12, if that's the most recently installed version of FD on the computer.
There's no need to copy and paste any content from a script into a blank FD12 document.
If you have FD12 installed and when you double-click on an FDX script icon it opens in FD11 / 10 / 9 / 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 FD12, go to File > Open and open older scripts that way.
2019
https://fd.support/42xLyT8