For the future, I wonder how useful it would be to run old hashed passwords through a newer system such as scrypt. This way those users who haven't logged in in awhile could also benefit from the safer hashed passwords.

    scrypt(hmac_sha1(password, salt), salt, cpumemargs)

In the future, you could even do it again with more cpu and memory requirements for scrypt, upgrading older users' hashes again with another run of scrypt.