PSA for those foolish enough to use Google Cloud and try to use private service connect: If you want to change the serviceAttachment your private service connect forwarding rule points at, you must delete the forwarding rule and create a new one. Updates are not supported. I've done that in the past via terraform, but lately encountered strange errors like this:

Error updating ForwardingRule: googleapi: Error 400: Invalid value for field '':
k8s1-sa-xyz-abc>'. Unexpected resource collection 'serviceAttachments'., invalid

Worked around that with the help of terrraform_data and lifecycle:

resource "terraform_data" "replacement" {
    input = var.gcp_psc_data["target"]

resource "google_compute_forwarding_rule" "this" {
    count   = length(var.gcp_psc_data["target"]) > 0 ? 1 : 0
    name    = "${var.gcp_psc_name}-psc"
    region  = var.gcp_region
    project = var.gcp_project

    target                = var.gcp_psc_data["target"]
    load_balancing_scheme = "" # need to override EXTERNAL default when target is a service attachment
    network               = var.gcp_network
    ip_address            =

    lifecycle {
        replace_triggered_by = [

See also terraform data for replace_triggered_by.