Use PHP 5.5's new password_* functions (or fall back on Anthony Ferrara's password_compat).
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README.md
wp-password-hash.php

README.md

WP Password Hash

Use PHP 5.5's new password_* API or fall back on Anthony Ferrara's password_compat library.

Should be backwords compatible. Users will get new password hashes when the log in.

Requirements

PHP 5.3.7+. The plugin does a check for this. if the version requirement isn't met none of the functions will be replaced.

Changing the Cost Factor

Hook into wp_password_hash_cost. If the cost is different from the previous cost, it should trigger a rehash of the password when the user next logs in.

add_filter('wp_password_hash_cost', function () {
    return 16;
});