I'm not sure they would force new transactions to use the new hashing algorithm. Bitcoin uses scripts (a little state machine) to embed verification behaviors d

I'm not sure they would force new transactions to use the new hashing algorithm. Bitcoin uses scripts (a little state machine) to embed verification behaviors directly into the blockchain. You could only introduce the new hashing algorithm to the protocol, and by natural incentive to upgrade (more security), expect more transaction outputs to use it. I'm not sure the Bitcoin developers would force clients to ignore certain hashing algorithms -- scripts can be very dynamic and the smaller the hashes are, the cheaper they are to add to the blockchain.