I'm no expert, but I think you could just convert a database via function composition:

    new_hash = (bcrypt new_work_factor) . old_hash -- new hashing function

    new_hashed_passwords = map bcrypt old_hashed_passwords -- convert the old hashed passwords to new

Of course, this will fail horribly if (bcrypt new_work_factor) is somehow an inverse (or partial inverse) of old_hash. It could also fail horribly if (bcrypt new_work_factor) maps it's input into a low "rank" (sorry, I'm a mathematician, not a crypto expert) region of old_hash's domain.