The Ruby on Rails auth system I use, Devise [1], will automatically update a user's password to use your new work factor on their next login. You could do some

The Ruby on Rails auth system I use, Devise [1], will automatically update a user's password to use your new work factor on their next login. You could do something similar. [1] https://github.com/plataformatec/devise https://github.com/plataformatec/devise