Don't forget to archive the class again afterwards, otherwise chaos ensues as those students start posting in their old class again!
https://help.seesaw.me/hc/en-us/articles/205002679-How-do-I-archive-or-delete-a-class-
You can browse the prior years' portfolios of your new incoming students on the Seesaw website.
To view past years' portfolios:
Sign in as a teacher at https://app.seesaw.me
Click on your profile icon (top left).
Click on Schoolwide Portfolios.
You will only have access to your current students’ portfolios.