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you have a swf that you want to decompile into a fla? if yes, you'll need a decompiler.
If you're talking about having the FLA and you want to open it and revert back to some older version of the FLA, no you can't do that. Managing versions of FLAs is up to you.
If you don't own the SWF then decompiling can be copyright infringement. If you do, it doesn't always work. There are many settings stripped from SWFs that FLAs have (graphics/audio compressed, etc).