As dijit (above this comment) has noted, this is somewhat possible and automated today.

For example, one method has the attacked IP get completely null-routed, and the subsequent route is advertised. Upstream routers will pick up the null-route advertisement and drop the traffic ever closer to the source(s). The effect of the null route is that the attacked IP is unreachable by anyone until the null-route is lifted... so the aim of the DDoS isn't averted, but at least the flood of traffic won't pummel any network paths except for (ideally) the paths between the attacker(s) and the first router respecting the null-route. In my experience the DDoS tends to stop more quickly and shift away to other targets if the folks directing the attack can no longer reach the target (because: null-route) and then the null-route can be lifted sooner relative to a long-running DDoS that hasn't shifted away to other targets.