How to convert Persistent Standard disk to SSD.

First create a snapshot of the disk.

gcloud compute disks snapshot your-server \
--snapshot-names manual-snapshot-disk-1 \
--project your-project \
--zone us-central1-c

Restore snapshot to SSD format.

gcloud compute disks create disk-1-ssd \
--source-snapshot manual-snapshot-disk-1 \
--project your-project \
--zone us-central1-a \
--type pd-ssd \
--size 10GB

Swap disks using GCP attach and detach.