The following procedure will help you to migrate virtual machines that are currently connected to port groups on a vSS to dvPortGroups on a vDS:

1. Connect to vCenter Server using the HTML5 client and go to the Networking inventory view.
2. Right-click on the vSphere Standard Port Group you would like to migrate the VM from and click Migrate VMs to Another Network:

3. On the Select source and destination networks screen, browse and set the desired dvPortGroup as the Destination network. Once you're done, click Next to continue: 

4. On the Select VMs to migrate screen, select All virtual machines or select only the VMs that you intend to migrate. Then, click Next to continue: 

5. On the Ready to complete screen, review the summary and click Finish: 

6. The Recent Tasks pane should list that the Reconfigure virtual machine tasks have been completed successfully:

This completes the process of migrating a virtual machine network from vSS to vDS.