The port group is responsible for the L2 (switching) communication between the VMs or for the VMK (management, vmotion, VSAN, .....) and it 's rely on the physical NIC's (must include uplinks ) specially if the two VM's in different hosts.
And for the Logical switch this is new in nsx-v and it's responsible for L2 trafific of the vms inside the nsx only (in the virtual environment) and the traffic between the distributed routers. its not depend on any physical network adapters and it's mainly responsible for the VXLAN packet. the logical switch here is only identifying the VXLAN segments.the portgroup in the NSX-V is responsible for the communication between the VTEPs for the physical communication between the hosts which including the VXLAN traffic.
SO, in simple way the portgroup is mandatory for the communication between the physical hosts for management or for the vtep communication portgroup understand only the VLAN traffic.
this was pref for the logical switch and the port group and to answer your questions:
1- Do we have to use a logical switch?
For the NSX yes it's must. as it is mandatory to identify the VXLAN segment ID.
2- what are Differences between a logical switch and a port group?
As mentioned previously
3- If we have to create logical switches, what do we do with the previous port groups that exist on VDS?
The Port Groups in the VDS will still work for the communication for the management network and you will use it for the VTEP network.
4- Can we use a port group as a logical switch?
No , we cannot use it . as the port group cannot understand the VXLAN packet.
I hope this pref answer your questions and i hope that this become answer for you also, for More details and more information just follow my blog http://www.syncgates.com.
if you're planning to move your workloads from VLAN to NSX logical switch (LS) so sure you will use LS.
LS is a virtual subnet under NSX, this subnet will not be created on your legacy network. the gateway (SVI) for this subnet will be on an NSX logical router.
The port group (PG) is VLAN tagged, so it is a translation for the normal VLAN that exists on the physical network to be used under the vSphere environment.
So the port group and NSX LS could be attached to a VM, the difference that LS is under NSX everything logical and managed by NSX components and use the VXLAN concept but PG forward the traffic to the physical network to use the legacy VLAN concept.