Modify existing redshift cluster

i’m pretty new to terraform. trying to update existing redshift cluster - specifically update the https://www.terraform.io/docs/providers/aws/r/redshift_cluster.html#iam_roles . trying to add one

i successfully import the resource and when hit plan it shows it’s going to destroy it and re-create it. am i missing something here? it’s impossible to modify this redshift parameter?

here’s my code:

resource “aws_redshift_cluster” “default” {
cluster_identifier = “{local.redshift_cluster}" node_type = "dc2.large" iam_roles = ["arn:aws:iam::{var.aws_account_id}:role/RedshiftS3ReadOnly”,"${aws_iam_role.rs_copy_unload_role.arn}"]
}

anyone?? thanks