This question already has an answer here:
If you already have SHA1 hashed passwords and you're trying to migrate to a better password hashing algorithm, is it less secure to just use PBKDF2 to hash the SHA1 hashes you already have instead of migrating users over when they next log in and hash the password with PBKDF2 directly?
In other words, is there any difference between these two approaches?:
The ability to migrate the existing hashes over in one shot would make a migration much easier.