Replacing grip shift shifters on department store bike with thumb friction shifters? I have a department store bike with a twist grip shifter (3 chain rings, friction shift and 6 sprockets, index shift - Torque Drive One derailleur) and I want to replace it. I am not able to get sufficient advice on bicycle forums because it is a Bicycle Shaped Object apparently and not worth any money on fixing. 

I have my reasons, and basically I am looking at something like http://amzn.to/papmPF or http://amzn.to/osZcI0 .. Is it possible to replace my current twist grip shifters with this? Is it a difficult process? 

I particularly want a friction shifter, so is this possible?
I've done it, on donated bikes we were rehabbing for Christmas Anonymous.  (Those twist grip shifters seem to fail pretty quickly.)  Don't recall any special problems, other than you have to refigure the cable routing, and you may need a different length cable.