Is there already a best practice white paper with all steps needed to downgrade from Nintex Workflow 2013 Enterprise to Standard license?
don't think there is a white paper that describes the process. I'd suggest you add a standard license to the staging environment and test all your workflows on a staging environment first. If they use enterprise features they will break.
Otherwise you could disallow all enterprise actions that are currently activated in your farm.
Afterwards you should test your workflows if they are still running good and if not you would have to remove enterprise functionality from your workflows or replace it with workarounds.
Once you stripped your workflows you can move them back to the production environment and add your standard license there.
A problem could be running workflow instances of old workflows that still know the enterprise functionality. Not sure how they will perform.
I'm don't know your motivation for downgrading but I think it's better to see how you get most out of enterprise consumption as you already paid for it rather than going though all this effort described above.
Hope this helps a bit.
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