I think it could be done, assuming there's enough time between the old hash looking vulnerable and it actually being broken:

Release a new client version X which uses the new hash after some future block N. Once block N+1000 has been found , hash every block up to N using the new hash and bake a final result of that into client version X+1, such that it rejects all old-hash blocks that haven't been blessed by the new hash.

Still, that is rather involved, and your destroy-to-convert scheme (which could be disabled once the old hash is looking too shaky) looks like it would work pretty well.

I'm not sure how well selling old coins and buying in would work, though - someone's going to be left holding a large bag of worthless bitcoins at the end of that.