With the release of NSX-T 3.2.0.1 customers can now upgrade to this version safely. This release came to mitigate some upgrade issues which prevented brownfield NSX-T customers in upgrading to NSX-T 3.2.
VMware has offered a upgrade evaluation tool which runs and check the current NSX-T deployment and evaluates the upgrade process to NSX-T 3.2.0.1 and if there might be any issues that can be discovered beforehand.

The tool is an OVA appliance that needs to be downloaded from VMware website and then deployed in your environment.

Download NSX Upgrade Evaluation Tool

Login to your VMware account and navigate to All Products > Networking & Security > VMware NSX-T Data Center and click on View Download Components

Click on Go To Downloads on the right of VMware NSX-T Data Center 3.2.0.1

Scroll down till the NSX Upgrades section and download the tool

Deploy NSX Upgrade Evaluation Tool

The tool is an OVA that you need to deploy as any other OVF template. So, navigate to your vCenter UI and right click the cluster on which you want to deploy the tool to and choose deploy OVF Template.

Choose the .ova file of the evaluation tool which was downloaded and click Next.

Go through the deployment wizard and set admin and root passwords and ensure that your networking configuration for the OVA is correct

Once the OVA is deployed, before powering it on, remove any resource reservations from the VM if you limited lab resources as myself 🙂

Right click VM > Edit Settings

Running the tool and evaluate the upgrade process

Power On the virtual appliance and open a web console to it and enable SSH

Next step, ssh to the OVA using root (you need to have SSH enabled for root) and run the following command to start evaluating an existing NSX-T deployment to be upgraded to NSX-T 3.2.0.1

> start dry-run data-migration mp-ip 192.168.50.1
Root password of the Remote MP node:
Checking ssh connectivity to the MP node 192.168.50.1 with root user...
Checking if nub is unpacked
Untaring nub file
Unpacking nub bundle
Nub unpacked
Creating a temporary folder on MP 192.168.50.1
Copy Corfu data to the temporary folder on MP 192.168.50.1
Copy nsx_issue file to the temporary folder on MP 192.168.50.1
Create tar of the temporary folder on MP 192.168.50.1
Delete the temporary folder on MP 192.168.50.1
Fetching tar containing Corfu data
Delete tar file on MP
Downloaded corfu tgz file of size 36 MB
Loading the fetched Corfu data
Stopping Corfu server
Stopping CBM
Preparing config_bak on this VM
Untaring the data
Starting Corfu server
Starting data-migration dry-run
Running.... Please track progress in /var/log/cloudnet/data-migration.log, /var/log/proton/data-migration.log, /var/log/policy/data-migration.log, /var/log/proton/logical-migration.log

All pre-upgrade check(s) passed. You can proceed with the upgrade.