This blog post has been heavily leveraged but unfortunately is outdated. Please refer to the following Knowledge Base article for further guidance:
Updating Workflow and Forms for SharePoint On-Premises
I thought I'd share my experience with upgrading (from 220.127.116.11 to 18.104.22.168) and, in particular, the issues I encountered and how I went about resolving them.
Firstly, at step 3 (above), the Solutions Management page was reporting the nintexforms2016.wsp and nintexforms2016core.wsp solutions as Error. The suggested fix is to select the solution, then click the Deploy Solution. However, when I went into my failed solutions I had no such option. Instead, I could Retract Solution -- which I did -- and once the retraction completes I was able to Deploy Solution. Once the deployment completed, both solutions were successfully reporting as Deployed.
However, I still had issues as no Nintex forms were working. For example, when attempting to edit/design a form, I was getting the following error:
The Expression prefix 'NFResources' was not recognized. Please correct the prefix or register the prefix in the <expressionBuilders> section of configuration.
To resolve this, I needed to re-activate the Nintex Forms feature on the web application(s) as suggested by this blog post. I don't know if this is normal practice for an upgrade (i.e. the feature gets deactivated) or a consequence of the error I experienced. Hopefully this post might help someone in future, and/or perhaps this document could be updated with the additional information on troubleshooting the error, if deemed appropriate.