title: Symfony2 - Change encoders for existing users
body: <p>If you have an existing code base with users and their passwords, how can you change the password encoder and have users' passwords update?</p>
  
  <p>In other words, let's say all user passwords are in MD5 and you want to transition to PBKDF2. The common strategy is to simply re-hash the password whenever the user logs in next.</p>
  
  <p>However, I'm not sure how to do this in Symfony. Would it be done in the login controller? Or is there a way to do it in the EncoderInterface object?</p>