This is possible, but the current VPS you are using will not be the one that is upgraded.
You will need to purchase a new VPS on the Fully-Managed tier which will be provisioned using a specific “Fully-Managed” profile so that it’s running our set configuration from the get go. No custom configurations or applications outside of the ones we provide will be carried over.
Backups will be taken of the cPanel accounts from your Self-Managed server and applied on to the Fully-Managed server.
Once you are all satisfied that it’s been brought across you will then be free to repurpose the Self-Managed server or if no longer required you can submit a cancellation request to have it closed down. Any time paid can be refunded and applied as a credit against your new Fully-Managed VPS.