Hi, the best converter from swf to fla I know is "Sothink SWF Decompiler".
You can find it here:
http://www.sothink.com/product/flashdecompiler/[
^]
It is great and matured software, has plenty of features. I used it myself for various situations, you can download it for Windows or Mac as 30 days trial version and try yourself.
I hope it helps ;) Good luck!