Turns out , even connecting the 3.5 Hosts into the 5.1 vCenter is not supported ( InterOP ) , so there goes any type of migrate / vMotion / SvMotion options .
Also the other option being Vmware Coverter simply wont work with 1300 VMs . Too Risky and downtime per VM is a long as the VM is big .
So will test this . PLEASE let me know if there is a better way .
Lets hope a VM with Virtual Hardware 4 can have a DVportgroup bound to it , or else i will have to update VMtools and vHardware before granting it Network access:-( longer downtime per VM