That's an interesting one; I dont think I've ever seen this before. They are saying that before they registered the debt and sent out an Order for Recovery, they checked the address again , and if a new one comes up, presumably with the same keeper, they then send out a fresh PCN. They don't say, but presumably cancel the Charge Certificate. I'm not sure they are following due legal process, so hopefully CP8759, or Neil B know more.
To re-check the keeper address they will have made a second enquiry of the DVLA for the reg. number of the vehicle.