Add support for modifying Transit Gateway ASN

.changelog/27306.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+resource/aws_ec2_transit_gateway: Add support to modify `amazon_side_asn` argument
+```

internal/service/ec2/transitgateway_.go

@@ -53,7 +53,6 @@ func ResourceTransitGateway() *schema.Resource {
 			"amazon_side_asn": {
 				Type:     schema.TypeInt,
 				Optional: true,
-				ForceNew: true,
 				Default:  64512,
 			},
 			"arn": {
@@ -233,6 +232,12 @@ func resourceTransitGatewayUpdate(d *schema.ResourceData, meta interface{}) erro
 			TransitGatewayId: aws.String(d.Id()),
 		}
 
+		if d.HasChange("amazon_side_asn") {
+			input.Options.AmazonSideAsn = aws.Int64(int64(d.Get("amazon_side_asn").(int)))
+		}
+
+		//aws.Int64(int64(v.(int)))
+
 		if d.HasChange("auto_accept_shared_attachments") {
 			input.Options.AutoAcceptSharedAttachments = aws.String(d.Get("auto_accept_shared_attachments").(string))
 		}

internal/service/ec2/transitgateway_test.go

@@ -241,7 +241,7 @@ func testAccTransitGateway_AmazonSideASN(t *testing.T) {
 				Config: testAccTransitGatewayConfig_amazonSideASN(rName, 64514),
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckTransitGatewayExists(resourceName, &transitGateway2),
-					testAccCheckTransitGatewayRecreated(&transitGateway1, &transitGateway2),
+					testAccCheckTransitGatewayNotRecreated(&transitGateway1, &transitGateway2),
 					resource.TestCheckResourceAttr(resourceName, "amazon_side_asn", "64514"),
 				),
 			},

website/docs/r/ec2_transit_gateway.html.markdown

@@ -23,6 +23,9 @@ resource "aws_ec2_transit_gateway" "example" {
 The following arguments are supported:
 
 * `amazon_side_asn` - (Optional) Private Autonomous System Number (ASN) for the Amazon side of a BGP session. The range is `64512` to `65534` for 16-bit ASNs and `4200000000` to `4294967294` for 32-bit ASNs. Default value: `64512`.
+
+-> **NOTE:** Modifying `amazon_side_asn` on a Transit Gateway with active BGP sessions is [not allowed](https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_ModifyTransitGatewayOptions.html). You must first delete all Transit Gateway attachments that have BGP configured prior to modifying `amazon_side_asn`.
+
 * `auto_accept_shared_attachments` - (Optional) Whether resource attachment requests are automatically accepted. Valid values: `disable`, `enable`. Default value: `disable`.
 * `default_route_table_association` - (Optional) Whether resource attachments are automatically associated with the default association route table. Valid values: `disable`, `enable`. Default value: `enable`.
 * `default_route_table_propagation` - (Optional) Whether resource attachments automatically propagate routes to the default propagation route table. Valid values: `disable`, `enable`. Default value: `enable`.