Question

How do I save a Flash15 compression swf to a Flash9 compression swf?

Answer 1

If you just want to change the version number, you could open your SWF in a hex editor and change byte number 4 from 0x0B (11) to 0x0A (10).
Link: https://dl.dropbox.com/u/59138954/Screen%20Shot%202012-08-14%20at%2012 .42.52%20PM.png
Or, try to use a decompiler : https://www.sothink.com/product/flashdecompiler/