There isn’t a built-in automatic option in Stash graphical interface. But in can be easily done programatically using API.
Note
Make a backup of your database first so that you can revert these changes if needed.
Guide: How to make a backup
Run the below code snippet in your GraphQL playground at http://localhost:9999/playground.
mutation {
execSQL(sql: "UPDATE scenes SET organized = false") {
rows_affected
}
}