Yes! You can use Clever this year, even if you have used CSV Rosters to create classes in the past.
When Clever and Seesaw sync for the first time, Clever will search for existing students to update their accounts and enroll them in their new classes. In order for Clever to find existing students in Seesaw, the Student ID field in Seesaw must match the SIS ID field in Clever. Seesaw can not match on Student Number or State ID fields.
Optionally, if your students have email addresses connected to their accounts in Seesaw, that will serve as a unique identifier for Clever. The sync will be able to locate those existing accounts and update their accounts.
If your school or district is already using Clever with other programs, please refer to the Creating Classes with Clever Sync to start the integration process.