Here’s how to move a VM to another VPC.

Stop VM.

gcloud compute instances stop $INSTANCE_NAME \
--zone $ZONE_NAME \
--project $PROJECT_ID

Move VM to another VPC.

gcloud compute instances network-interfaces update $INSTANCE_NAME \
--zone $ZONE_NAME \
--network-interface=nic0 \
--network $YOUR_NETWORK \
--subnetwork $YOUR_SUBNETWORK \
--project $PROJECT_ID

Start VM.

gcloud compute instances start $INSTANCE_NAME \
--zone $ZONE_NAME \
--project $PROJECT_ID