hi jagdish_rana,
Thank you to confirm it works
so at the point you attach the host with the running VMs on it to the new vcenter shows any problem?
afterwords just de-attach the grayed-out esxi from 4 will be enough.
even virtual hardware version 4 is compatible with both esxi5 and vcenter 5.5 so i think there will be no problem to keep it running couple of weeks.