I've stuffed up deploying vRNI. I mistook the reference to a cluster deployment as describing the vSphere environment that I am going to analyse. So I've ended up deploying a "Large" vRNI platform brick.
The VMware Knowledge Base article on this shows that its pretty easy to scale up (from Medium to Large).
Is it just as easy to reduce the platform (i.e. Power Off, reduce vCPU to 8 and RAM to 32 GB)?
Yes, it is as easy as shutting down the appliance and giving it the wanted virtual hardware. When you start it back up, it will recognise the new size and adjust its services accordingly. Size them according to the brick sizes, that are documented here: System Recommendations and Requirements