Regarding migration check this one https://docs.vmware.com/en/VMware-vCloud-NFV/3.2/GUID-702EF8D4-6AA5-42BD-B334-AEA7F7BA14CC-1-en.pdf
yes this is not an easy task, check on this
it is not possible to have the same ESXi prepared with the vibs of NSX-v and NSX-t what is possible is to have the same environment with v and t separated just by the clusters prepared for nsx-v or t and nsx manager is not dependent of vCenter so that can be possible.
Just notice that NSX as part of NFV it is not the same as NSX-T "flat deployment", take in account the considerations for the NFVis and such, if you have enough room for having a new cluster with NXS-T I would say that a good strategy for migration could be to pivot VNF's from one environment to another on the same vSphere just be cautious on details like DPDK etc. and be sure of N-s routing from this greenfeld NSX-T environment.
hope this helps you.