Maybe one approach that you can try is use some replication tool for the VMs and prepare the NSX-T platform with all the necessary construction and the BGP disabled. After you do the first replication you can trigger the switch to the new platform.
Of course this will involve stopping the redistribution from the NSX-V platform or at least using prefix for limiting that subnet and disconnect the DLR or Edge interface for enabling it in the Tier1 or Tier0. This method however involves a downtime and a maintenance windows but you can easily prepare this environment with some tests networks to try all the procedure.
Also I was thiking that maybe you can configure a L2VPN between NSX-V and NSX-T as they allow VXLAN to VXLAN but I am not sure this is supported as NSX-T uses Geneve for the Overlay protocol.
Hi. Thank you very much. I am going to bring this up, this in an option we will discuss I am sure. Thanks,,,