allow to add subnets/ mappings to a lb

.changelog/33205.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+resource/aws_lb: Allow to add subnets without forcing recreation
+```

internal/service/elbv2/load_balancer.go

@@ -216,18 +216,15 @@ func ResourceLoadBalancer() *schema.Resource {
 				Type:     schema.TypeSet,
 				Optional: true,
 				Computed: true,
-				ForceNew: true,
 				Elem: &schema.Resource{
 					Schema: map[string]*schema.Schema{
 						"allocation_id": {
 							Type:     schema.TypeString,
 							Optional: true,
-							ForceNew: true,
 						},
 						"ipv6_address": {
 							Type:         schema.TypeString,
 							Optional:     true,
-							ForceNew:     true,
 							ValidateFunc: validation.IsIPv6Address,
 						},
 						"outpost_id": {
@@ -237,13 +234,11 @@ func ResourceLoadBalancer() *schema.Resource {
 						"private_ipv4_address": {
 							Type:         schema.TypeString,
 							Optional:     true,
-							ForceNew:     true,
 							ValidateFunc: validation.IsIPv4Address,
 						},
 						"subnet_id": {
 							Type:     schema.TypeString,
 							Required: true,
-							ForceNew: true,
 						},
 					},
 				},
@@ -604,6 +599,19 @@ func resourceLoadBalancerUpdate(ctx context.Context, d *schema.ResourceData, met
 			return sdkdiag.AppendErrorf(diags, "failure Setting LB Subnets: %s", err)
 		}
 	}
+	if d.HasChange("subnet_mapping") && !d.IsNewResource() {
+		params := &elbv2.SetSubnetsInput{
+			LoadBalancerArn: aws.String(d.Id()),
+		}
+		if v, ok := d.GetOk("subnet_mapping"); ok && v.(*schema.Set).Len() > 0 {
+			params.SubnetMappings = expandSubnetMappings(v.(*schema.Set).List())
+		}
+
+		_, err := conn.SetSubnetsWithContext(ctx, params)
+		if err != nil {
+			return sdkdiag.AppendErrorf(diags, "failure Setting LB Subnet Mapping: %s", err)
+		}
+	}
 
 	if d.HasChange("ip_address_type") {
 		params := &elbv2.SetIpAddressTypeInput{
@@ -1040,12 +1048,18 @@ func customizeDiffNLB(_ context.Context, diff *schema.ResourceDiff, v interface{
 	// Get diff for subnets.
 	o, n := diff.GetChange("subnets")
 	os, ns := o.(*schema.Set), n.(*schema.Set)
-
-	if add, del := ns.Difference(os).List(), os.Difference(ns).List(); len(del) > 0 || len(add) > 0 {
+	if del := os.Difference(ns).List(); len(del) > 0 {
 		if err := diff.ForceNew("subnets"); err != nil {
 			return err
 		}
 	}
+	o, n = diff.GetChange("subnet_mapping")
+	os, ns = o.(*schema.Set), n.(*schema.Set)
+	if del := os.Difference(ns).List(); len(del) > 0 {
+		if err := diff.ForceNew("subnet_mapping"); err != nil {
+			return err
+		}
+	}
 
 	// Get diff for security groups.
 	o, n = diff.GetChange("security_groups")

website/docs/r/lb.html.markdown

@@ -123,7 +123,7 @@ Terraform will autogenerate a name beginning with `tf-lb`.
 * `preserve_host_header` - (Optional) Indicates whether the Application Load Balancer should preserve the Host header in the HTTP request and send it to the target without any change. Defaults to `false`.
 * `subnet_mapping` - (Optional) A subnet mapping block as documented below.
 * `subnets` - (Optional) A list of subnet IDs to attach to the LB. Subnets
-cannot be updated for Load Balancers of type `network`. Changing this value
+can only be added for Load Balancers of type `network` (see [Availablity Zones](https://docs.aws.amazon.com/elasticloadbalancing/latest/network/network-load-balancers.html#availability-zones)). Deleting a subnet
 for load balancers of type `network` will force a recreation of the resource.
 * `xff_header_processing_mode` - (Optional) Determines how the load balancer modifies the `X-Forwarded-For` header in the HTTP request before sending the request to the target. The possible values are `append`, `preserve`, and `remove`. Only valid for Load Balancers of type `application`. The default is `append`.
 * `tags` - (Optional) A map of tags to assign to the resource. If configured with a provider [`default_tags` configuration block](https://registry.terraform.io/providers/hashicorp/aws/latest/docs#default_tags-configuration-block) present, tags with matching keys will overwrite those defined at the provider-level.