Which is the one piece grip, the new or the old? The way I read it, the grip that is on the gun is one-piece. If so, you have to remove the backstrap to remove them. You would also have to remove the backstrap to put one-piece grips on, and also remove the stock pin.
If the new grips are one piece, I don't understand Oldjarhead's advice. One piece grips don't/can't use a stock pin, and if one is installed it has to be removed before installing one-piece grips. If a stock pin is in there and the grips fitted before gluing them, the grips can never be removed without breaking them off.