Using Configurator 2, Prepare Device Without Changing iOS Version
It seems that the way Apple wants you to do this is described here:


  Update iOS beta software 
  You can use Apple Configurator 2 to install beta versions of iOS software.
  
  
  To install iOS beta software on a device, select the device in the    device browser and drag the .ipsw file onto the device.
  




However, the specific use case described is for beta versions of iOS. In my case, I wanted to install an older (than the newest public) version of iOS. It looks like the UI is the same for doing either. 

When I attempted to prepare my device this way, the process failed with error codes: 


  Personalization failed: 4027  
  Personalization failed: 3194


I am not clear whether this is because updating a device to less than the newest version is unsupported ... or there is some other problem. Therefore, I'm leaving this answer, but not accepting it as solved, should someone else have more to add.