Reference this: VMware vSphere 5.1 - "Service Console virtual NICs (vswifs) will be converted to ESXi virtual NICs. (vmks)"
"The upgrade process preserves as much of the ESX host configuration as possible. However, because of the architectural differences between ESX 4.x and ESXi 5.x architecture, many configuration files cannot be migrated when you select the Migrate option in the ESXi installation or upgrade wizard.
Pertinent VMware files, such as /etc/vmware/esx.conf are migrated, but many existing settings such as third-party agents and scripts, cannot be migrated."
Good luck!