you can publish the swf from cs5 to a flash player 8 versions.
but if you want to open the cs5 fla in cs2 you'll need to save the fla (in cs5) for cs4. then you'll need to open the fla in cs4 and save for cs3. then you'll need to open the fla in cs3 and save for flash 8.
not very convenient, i know.
or, you could use a decompiler and decompile the cs5 swf to a flash 8 fla directly.
If the CS5 file uses AS3 you're sol.