This is a good practical question. My read of the algorithm is that you must force each user to enter a new password, and encrypt that at the new higher cost.
If you wanted to "upgrade" the passwords to the higher cost key schedule, you'd just continue the key schedule where it left off--but this would require knowing the original password! So that's not really an option.
If you wanted to "upgrade" the passwords to the higher cost key schedule, you'd just continue the key schedule where it left off--but this would require knowing the original password! So that's not really an option.