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here's the source code for the resource:
```
const vpnInstance = new gcp.compute.Instance(
    'instance',
    {
        name: 'vpn-00a',
        zone: 'us-west2-b',
        machineType: 'n1-standard-1',
        canIpForward: true,
        deletionProtection: true,
        metadata: { 
            'startup-script-url': startupScriptURL
        },
        scheduling: {
            automaticRestart: true
        },
        bootDisk: {
            initializeParams: {
                image: 'ubuntu-os-cloud/ubuntu-minimal-1804-lts',
            },
        },
        networkInterfaces: [{
            subnetwork: vpnSubNetwork.id,
            networkIp: vpnInternalStaticIP,
            accessConfigs: [{
                natIp: vpnExternalIP.address
            }]
        }],
        serviceAccount: {
            scopes: ['<https://www.googleapis.com/auth/cloud-platform>'],
        },
    },
    { dependsOn: [vpnFirewall] });
```

Debugged this with <@UCPSHLVC4>.  Turns out GCP provider made a change recently, and you now need to use  `subnetwork: vpnSubNetwork.selfLink` instead of `subnetwork: vpnSubNetwork.id` here.

He was getting the confusing diff shown in <https://pulumi-community.slack.com/archives/C84L4E3N1/p1563404171471600> because he had `0.17.22` of the `pulumi` CLI, but in `0.17.23` we actually released an improved rendering of this diff - and with that he gets a clearer presentation of what the root cause of this problem is:

```
   +-gcp:compute/instance:Instance: (replace)
        [id=vpn-00a]
        [urn=urn:pulumi:gcp-project-solvvy-prod::gcp-project-solvvy-prod::gcp:compute/instance:Instance::instance]
      ~ networkInterfaces: [
          ~ [0]: {
                  ~ subnetwork: "<https://www.googleapis.com/compute/v1/projects/solvvy-production/regions/us-west1/subnetworks/vpn-subnetwork>" =&gt; "us-west1/vpn-subnetwork"
                }
        ]
```