ok, first of all, you wont be able to rethread the shifter if u cut all of the threaded bit off. because the gearstick goes bigger in diameter as it goes down right ??? which means u actually gotta turn "machine" it down to size.
example lets say the thread for the gear knob is a M12 thread, means that the diameter of that thread is 12mm.
so meaning that if u wanted to rethread the gearstick, the die nut wont fit on because u need to machine it down to 12mm.

but id suggest just go invest into proper short shifter man, sure cutting it makes it feel like the throw is shorter, but ur not actually changing much.

its actually the bottom bit that u have to modify to make the actual throw shorter