The topic author is not suggesting to "sunset" coins for security reasons, but for some bullshit semi-inflational reasons, which is plainly wrong and destroys the sancrosanct rules of Bitcoin as Gavin said.
Once the cryptography behind Bitcoin gets broken, I am sure that majority will agree that something needs to be done about old, insecure coins. In that case, i suppose it would be possible to re-hash the old coins somehow with new algorithm by sending them to a upgraded address using upgraded client.
This. When (or more likely) if ECC is compromised the most likely scenario (based on prior crypto attacks) it likely will be a partial compromise. That is private keys must be brute forced but can be done so millions or maybe even billions of times faster. That means that the day 0 threat to Bitcoin will be minimal. The network will operate for a while with "legacy" addresses and new "strong" addresses.
In time though those legacy addresses will represent a risk. A method to sunset that risk may need to be devised. Even then it will be controversial and require some thought. To change the protocol today in such a fundamental manner is just asinine.