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.