I think this is good practice. It means that if you upgrade your hashing function (to something more secure) new users will use the one new one immediately.
Now, you can't rehash users on the old hashing functions immediately, since you need their password to do that, and you can't retrieve it from its hashed state. Instead, when such a user logs in, you use their password to store a new hashed column, and reset the hashing function against their user account.
Thus, with such a configuration, users will slowly move onto the more secure system as they log in.