They can change all or just part of the system for a revision. It really depends on the situation, but generally they won't mess with something working and they can test various components during a revision to see if something is not working. For infections the whole system can be removed, infection treated (leaving lots of details out), new system put in. For blockages in the catheter draining fluid from the ventricle might just replace the catheter (one of the more common ways things can go wrong). We've had valve replaced two or three times, proximal catheter replaced once, and things added (nothing replaced that time, I think). Our child still has the original tubing that leads from the valve to the abdomen.