From time to time, I use the incredibly powerful Migrate module to migrate a subset of users from one Drupal 7 site to another.
Setting up the user migration class is pretty straightforward, and there are some great examples out there for the overall process. However, I couldn't find any particular documentation for how to preserve user passwords when migrating users from D7 to D7. It's simple enough to set the 'md5_passwords' boolean for Drupal 6 to Drupal 7 user migrations, so passwords will be updated when a user logs in the first time on the D7 site... but it's not as straightforward if you want to simply move the salted/hashed passwords from D7 to D7.
During the migration, when the user account is saved, Drupal will re-salt and re-hash the already-hashed-and-salted password you pass in through your field mappings, and users will have to reset their passwords to log in again.
To override this behavior, you need to implement the
complete() function in your user migration, and manually overwrite the just-saved user account password field:
// Reset password hash back to the source hash; when Migrate saves the user
// entity after the prepare() method is complete, Drupal hashes the hash,
// meaning we need to set it back to the original hash.
// SEE: https://drupal.org/node/1349758
->fields(array('pass' => $row->pass))
Granted, there are far fewer Drupal 7 -> Drupal 7 user migrations happening than migrations from Drupal 5 or 6 to 7, or other systems into Drupal 7... but I wanted to post this so someday, when I'm scratching my head over this same issue again (why are my hashes changing???), I can find the answer in a few seconds via Google :)