Manual:WrapOldPasswords.php/en

Details
wrapOldPasswords.php file is a maintenance script to wrap all passwords of a certain type in a specified layered type that wraps around the old type.

Since MediaWiki 1.24, if using the default settings, password hashes will automatically be updated to PBKDF2 as users log in. If necessary, the old MD5 hashing can be restored by changing  to 'B'. wrapOldPasswords.php can wrap all passwords in pbkdf2-legacyA or pbkdf2-legacyB (or the hashing algorithm of your choice) if you don't want to wait for your users to log in.