Thanks to the quick response of the Solarwinds support team this problem is now solved.
The issue was that I had an old version of the Solarwinds License manager (220.127.116.11
) which was not detecting the new Netflow product.
Once I downloaded version (18.104.22.1680) the products were detected & the upgrade option was now visible.
The process was smooth afterwards, I just had to add the activation code & re-run the Configuration Wizard & Netflow was up & running immediately.