Now imagine what happens if some nodes have the patch and some don't, and it's possible to convince the ones that don't to forward the garbage they receive. It wouldn't have to be enough garbage to pose a denial-of-service threat in itself, just enough to trigger other nodes to shun the unpatched part of the network.
Most of the garbage we're talking about wouldn't be forwarded (such as orphaned blocks). But you pose a very good point if the attack comes in the form of something that does cause node-to-node traffic.It might be necessary to implement some of these changes with a trigger to implement them only after a certain block number is received. We then leave enough room for at least most of the network to update.
Another possible idea is to make a node submit some proof of work to connect to you. You can normally demand none or a very small one. But if you sense an attack, start demanding proof of work and start disconnecting nodes that place an unusual load on you. That would increase the computational resources an attacker would need, and it wouldn't affect existing non-malicious node-to-node connections.
