Oh, I think I misunderstood your comment. I thought you were referring to third-party auth specifically.
Another option more on the cheaper side would be a library that handles algorithm selection and migration in a "black box" way. I.e. the public interface is only `db.put(user,newPass)`, `db.compare(user,suppliedPass)` and maybe something like `db.doMaintenance()`. You would need to handle storage yourself in that case but I don't see the downside of this when you are storing your users' data anyways. I'm not aware if such a library exists though.
