I\'m just digging into Symfony2 and just got my own user-provider running. ATM I use brypt with a cost of 12. If I now increase the cost, bcrypt should rehash the password again
You can change the cost in any moment because as you can read in the official symfony2 docs you don't need to rehash the old passwords because they are automatically handled with the old cost (and if you want you can force the users in the future to change their password like happens in many large sites).