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

Stop the VM.

gcloud compute instances stop server-name \
--zone=us-central1-c \
--project project-id

Migrate the VM. Use self link for network and subnetwork.

gcloud compute instances network-interfaces update server-name \
--zone=us-central1-c \
--network-interface=nic0 \
--network=your-network \
--subnetwork=your-sub-network \
--project project-id

Start the VM.

gcloud compute instances start server-name \
--zone=us-central1-c \
--project project-id