You'll probably need the right tool for a thread that size, it must be ~35mm in diameter And fitted by a machine, and not really designed to be serviced!
And i don't think you can simply change a mega range freewheel to a non-megarange by changing the sprockets, i have a vague recollection that the top "cluster" is riveted together into one piece and designed to fit to that specific freewheel body.........