Now that is an idea, it could easily be implemented with soft-forks.
Having said that, what is clear is that at some stage quantum resistant Bitcoin has to come in to force and become a reality. As mentioned in one of the links in this thread, it might take 5 years maybe 10 but eventually they will get access to private keys.
If it did come down to it I honestly cannot see anybody complaining about a hard fork if it was a simple choice between the end of Bitcoin or it carrying on (but those who did not move their coins before any fork just might have a differing view).
it could even be done with soft forks---one soft fork to implement a post-quantum signature scheme, and another to destroy all ECDSA-secured outputs after date
x.