I know it’s possible to extract these files from the game with OpenIV, modify headers and extension and then decompile them with various software like Sothink SWF Decompiler or JPEXS Free Flash Decompiler to get editable ActionScript files.
But how to compile them back into swf after making changes? I know it’s possible, because people sometimes release resources with custom, modified gfx that override the originals.