The vCenter upgrade is relatively simple and safe, but like any change, always take backup before start the process of upgrading. There is no problem if vCenter is a physical box, but if it was a VM a benefit will be take a snapshot of VM before start the vCenter.
About DVS to VSS migrating, you can do this even without downtime, just create a new standard switch, move some NIC to this VSS, create appropriate port groups and migrate VMs and VMkernels.
A final word about vCenter, if your physical box run a SO 32 bits, you will not able to do a in-place upgrade of vCenter, since the vCenter 4.1/5.x only runs on 64 bits SO.