What you can do is remove the port group then re-add it with the new pg name.
$adapters = Get-VMHost | Get-VMHostNetworkAdapter | ? {$_.portgroupname -match "FAB"}
foreach ($adapter in $adapters) {
Remove-VMHostNetworkAdapter $adapter -Confirm:$false if ($adapter.portgroupname -match "Fab-A") {New-VMHostNetworkAdapter -VMHost $adapter.vmhost -IP $adapter.IP -SubnetMask $adapter.subnetmask -PortGroup $newnfspga -VirtualSwitch $dvs} if ($adapter.portgroupname -match "Fab-B") {New-VMHostNetworkAdapter -VMHost $adapter.vmhost -IP $adapter.IP -SubnetMask $adapter.subnetmask -PortGroup $newnfspgb -VirtualSwitch $dvs}
}