As a piece of information:
I was using Simple Features on my test bed server. I decided to try out the new webGui. I read through the thread to this point, making notes on how to do it.
First, I backed up the entire flash drive to my windows computer.
Second, I deleted the contents of /boot/plugins (plugins in Windows).
Third, followed the instructions on the GitHub page. I copied and pasted the commands from the page into a terminal session using PuTTY.
Fourth, started the WebGui in my browser. At this point, Everything seemed fine. Then I noticed that the addins for Simple Features (Health, "SimpleFeatures Settings" on the Setting tab) were still there. However, the addins not longer worked that there were error messages on many of the pages.
I corrected this issue by rebooting the system. Apparently, even though I had deleted the files on the Flash drive, they were still in memory and were being used by the new webGui.