How to decompile a DCR
You can't directly convert a dcr, and there's no known way of getting the scripts out of a dcr, but you can get most other elements:

Import the dcr as a cast member.
Place it on the stage, in sprite 1, frame 1. Make sure the playback head is on that frame.

Next, open a script window, and paste this:
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on extractAssets

mov = sprite(1).movie

repeat with cn = 1 to mov.castlib.count
tCast = mov.castlib(cn)
repeat with mn = 1 to tcast.member.count
tMem = mov.member(mn,cn)
if tMem.type <> #empty then
tMem.copyToClipBoard()
newMem = new(#bitmap) -- type doesn't matter
newMem.pasteClipboardInto()
newMem.name = tMem.name
newMem.unload()
put "extracted "&tMem.name&" (mem "&mn&" castlib "&cn&" - "&tMem.type&")"
end if
end repeat
end repeat

end
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Next, check that the script type is set to 'movie' (in the property inspector).
Hit the little lightning button in the script window toolbar (recompile all scripts).

Now open the message window, and type:

extractAssets

Hit return, and voila!, your cast is filled with the cast members from the dcr. Any script cast members will be empty though. Can't help with that.

hope this helps,

50vil