[Stefan_H]

I would only say that you would want to immediately reset all user passwords if the passwords were leaked, not necessarily if the algorithm that you are using is bad for whatever reason. And the idea would be to give users a couple weeks (or days) to log in and then force the reset on all the remaining(maybe once you get to a certain percentage of your active user base).

I like the method that link provided, but there are some drawbacks, needing to update every user record with a new hash (offline process) - this is almost guaranteed to require taking the site down, which most people do not like to do. This is because you can't have some users with the old hashing process ,and some with the new.