We were able to get the migration to complete successfully!  Below are the note and changes we had to make to get the migration from NSX-V to NSX-T to work.

1. We were using a newer version of NSX-T that was not supported with the version of vSphere/vCenter we are using.  NOTE: always check the VMware compatibility matrix before starting a project like this!
2. During the initial connection from NSX-T to the compute manager (vCenter server), NSX-T showed an error for the ESX Agent Manager, "Failed to login to vCenter as extension, Cannot complete login due to an incorrect user name or password", ESX Agent Manager".  To fix this, we had to follow the steps described here: https://kb.vmware.com/s/article/2112577.
3. When performing the migration, we originally chose to do the Distributed Firewall, Hosts and Workload.  The problem with this option is that it does not create the N-VDS switches or segments needed (we aren't at vCenter 7.0 or later, so we need the N-VDS switches).  Instead, we chose the migration option "Migrate NSX for vSphere".  This option successfully created the N-VDS switches, created the segments, migrated the distributed firewall (DFW) rules, reconfigured the hosts, and migrated the workloads - all without any downtime.

Hopefully this helps someone else!