In GCP, you cannot directly move a Compute Engine instance to a different subnet. Instead, you’ll need to recreate the instance in the target subnet while preserving its data. Here’s how to do that using GCP Console, gcloud CLI, and Terraform:

1. Using GCP Console:

a. Go to the GCP Console: https://console.cloud.google.com/

b. Navigate to Compute Engine > VM instances.

c. Stop the instance you want to move by clicking the “Stop” button.

d. Click on the instance, and then click the “Create similar” button at the top of the page.

e. Change the subnet by clicking on the “Networking” tab and selecting the desired subnet under “Network interfaces.”

f. Optionally, assign a new internal IP address if needed.

g. Click the “Create” button to create the new instance in the target subnet.

h. Verify the new instance is working correctly, and then delete the old instance.

2. Using gcloud CLI:

a. First, stop the instance:

gcloud compute instances stop INSTANCE_NAME --zone ZONE

Replace INSTANCE_NAME and ZONE with appropriate values.

b. Create a new instance with a different subnet:

gcloud compute instances create NEW_INSTANCE_NAME --subnet TARGET_SUBNET --image-family IMAGE_FAMILY --image-project IMAGE_PROJECT --zone ZONE --tags TAGS

Replace NEW_INSTANCE_NAME, TARGET_SUBNET, IMAGE_FAMILY, IMAGE_PROJECT, ZONE, and TAGS with appropriate values.

c. Verify the new instance is working correctly, and then delete the old instance:

gcloud compute instances delete INSTANCE_NAME --zone ZONE

Replace INSTANCE_NAME and ZONE with appropriate values.

3. Using Terraform:

a. Update the main.tf file by modifying the existing google_compute_instance resource:

resource "google_compute_instance" "example" {
  # ... existing configuration ...

  network_interface {
    network = "TARGET_NETWORK"
    subnetwork = "TARGET_SUBNET"
    # ... other settings ...
  }
}

Replace TARGET_NETWORK and TARGET_SUBNET with appropriate values.

b. Run the following commands to apply the changes:

terraform init
terraform plan
terraform apply

Terraform will replace the existing instance with a new one in the target subnet. Ensure that you have a backup of the instance’s data before applying the changes, as replacing the instance may result in data loss if not properly backed up.

Keep in mind that moving an instance to a different subnet can cause the instance to lose its internal IP address. To preserve the IP address, you can reserve it as a static internal IP and assign it to the new instance. Also, remember to back up any data on the instance before deleting it.