My WAN Gateway 172.16.x.x range which has been the same for years has changed.
What causes this?
What is this the address of? My Cabinet? My Exchange?
No, the BRAS is not located at your local exchange or cabinet, they are located at a small number ( <100 )of sites across the country to aggregate local nodes.
If the reason for your concern is whether your broadband speed will change, the answer is definitely not as far as sync speed is concerned as that is determined by the distance of your copper cable to the DSLAM in the green cabinet. However, there is a slight possibility that the network has been re-arranged to overcome congestion problems in the backhaul so you might see an improvement in throughput speeds if you have been experiencing problems.
It could well be a result of the change, improvements to the network architecture. Nobody can change which BRAS you connect to without a major re-arrangement of the BTWholesale network.