update migration doc to mention issue with single count mvs

docs/upgrading_to_v2.0.md

@@ -48,7 +48,7 @@ Terraform will perform the following actions:
       - self_link                = "https://www.googleapis.com/compute/v1/projects/dev-xpn-networking/regions/us-west1/subnetworks/simple-project-timh-subnet-02" -> null
     }
 
-  # module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork["us-west1/simple-project-timh-subnet-01"] will be created
+  # module.example.module.test-vpc-module.google_compute_subnetwork.module.subnets.subnetwork["us-west1/simple-project-timh-subnet-01"] will be created
   + resource "google_compute_subnetwork" "subnetwork" {
       + creation_timestamp       = (known after apply)
       + enable_flow_logs         = false
@@ -65,7 +65,7 @@ Terraform will perform the following actions:
       + self_link                = (known after apply)
     }
 
-  # module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork["us-west1/simple-project-timh-subnet-02"] will be created
+  # module.example.module.test-vpc-module.module.subnets.google_compute_subnetwork.subnetwork["us-west1/simple-project-timh-subnet-02"] will be created
   + resource "google_compute_subnetwork" "subnetwork" {
       + creation_timestamp       = (known after apply)
       + enable_flow_logs         = true
@@ -93,11 +93,17 @@ can't guarantee that exactly these actions will be performed if
 
 ### Manual Migration Steps
 
-In this example here are the two commands used migrate the subnets created by the `simple_project` in the examples directory.  _please note the need to escape the quotes on the new resource_. You may also use the migration script.
+In this example here are the commands used migrate the vpc and subnets created by the `simple_project` in the examples directory.  _please note the need to escape the quotes on the new resource_. You may also use the migration script.
 
--   `terraform state mv module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[0] module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-01\"]`
+-   `terraform state mv module.example.module.test-vpc-module.google_compute_network.network module.example.module.test-vpc-module.module.vpc.google_compute_subnetwork.network`
 
--   `terraform state mv module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[1] module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-02\"]`
+-   `terraform state mv module.example.module.test-vpc-module.google_compute_subnetwork.subnetwork module.example.module.test-vpc-module.module.subnets.google_compute_subnetwork.subnetwork`
+
+-   `terraform state mv module.example.module.test-vpc-module.module.subnets.google_compute_subnetwork.subnetwork[0] module.example.module.test-vpc-module.module.subnets.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-01\"]`
+
+-   `terraform state mv module.example.module.test-vpc-module.module.subnets.google_compute_subnetwork.subnetwork[1] module.example.module.test-vpc-module.module.subnets.google_compute_subnetwork.subnetwork[\"us-west1/simple-project-timh-subnet-02\"]`
+
+*You'll notice that because of a terraform [issue](https://github.com/hashicorp/terraform/issues/22301), we need to move the whole resource collection first before renaming to the `for_each` keys*
 
 `terraform plan` should now return a no-op and show no new changes.
 
@@ -168,3 +174,23 @@ actions need to be performed.
     ```
 
 4.  Run `terraform plan` to confirm no changes are expected.
+
+### Known Issues
+
+If your previous state only contains a **single** subnet or route then `terraform mv` will throw an error similar to the following during migration:
+
+```
+Error: Invalid target address
+
+Cannot move to
+module.example.module.test-vpc-module-01.module.routes.google_compute_route.route["multi-vpc-a1-01-egress-inet"]:
+module.example.module.test-vpc-module-01.module.routes.google_compute_route.route
+does not exist in the current state.
+```
+
+This is due to a terraform mv [issue](https://github.com/hashicorp/terraform/issues/22301)
+
+The workaround is to either
+
+1. Create a temporary subnet or route prior to migration
+2. Manually updating the state file. Update the `index_key` of the appropriate user and push the to the remote state if necessary.