Upgrade sha1 => scrypt without user logging in/pw reset:
We hash passwords with one-way hash functions to reduce the impact of password database compromise. Passwords hashed with salted SHA or MD5 can ofen be brute forced. Newer password hashhing techniques like Scrypt/Bcrypt/PBKDF2 add computational work to make brute forcing prohibitively expensive.
Your service already has users with salted sha1 password hashes. Since you don't have their original passwords you can't upgrade to a modern password hash without getting each user to log in and enter their password.
You have (salt,sha(salt+password)), you want (salt, scrypt(salt,password))