I know know about Windows, but mac automaically uses the name of the present file. You get a conflcit if you try to put something in the same folder as the original, so I made the script pick the AI8 folder that I created and the whole thing runs.
If I have a doc named "picture.ai", when I export it the name that comes up witout typing is "picture.ai" and when I choose something like eps the name automatically becomes "picture.eps".
In the case of the script, it called for exporting "picture.ai" as a legacy .ai. That keeps the name as "picture.ai" without any intervention on my part, but the script changed the destination folder to "AI8".
Since it's better to keep your originals in your latest ILL version and only export to older when it's needed by a recipient, I don't think it's such a good idea to backsave all documents - replacing yur originals.