As the shaft is screwed, it doesn't really matter if you twist the shaft when extracting - this is what can destroy a graphite shaft when its being extracted.
Aside from the fact you can't use a piping cutter to cut graphite, the only other thing you need to concern yourself is the tip diameter. The G20 has a .350 diameter - so you either need to get a .350 shaft or a shim.
Should be a relatively easy job and a doddle if you've done irons before.
Did you buy it from somewhere like American golf? If so, take it back to them. I did after snapping it when shutting my boot lid and Ping replaced it free of charge.