You need to change the distribution file in the 10.13.1Update.pkg.
Modifying the 10.13.1 Update for High Sierra.
Download 10.13.1 Update.
Open 10.13.1 Update.
Move 10.13.1Update.pkg to Desktop.
Open Terminal.
Enter pkgutil --expand ~/Desktop/macOSUpd10.13Update.pkg ~/Desktop/Expanded
Open Expanded folder and then open Distribution file in Text Wrangler
Change Distribution file, change Board iD line 33 to <your Macs boardID> at line 41 remove <your Mac model>
Save As .xml file to the Desktop
In Terminal chmod +x ~/Desktop/distribution.xml
Remove the extension .xml
Move new Distribution file to Expanded folder replacing the original
In Terminal pkgutil --flatten ~/Desktop/Expanded ~/Desktop/Modified.pkg
Using the .pkg extension is crucial.
The Modified package on your desktop should now allow you to install the latest update
You Rock L Caputo!! Thanks so much! The new Modified.pkg seems to be working! well, almost, the installer made it 98% of the way thru, but then I get an error saying "The installation failed"

upon restart, it appears the update DID install and the system is running 10.13.1
the bad news- Safari doens't seem to connect (Firefox does), Mail doesn't show the contents of mail. app store still is hung on Updates..and who knows what else might not work.
now I def have to figure how to go back to 10.13 if I can. (any easy steps for that?)
But I think it all came close and may or may not work for others who encounter this situation, so I'll post some notes here:
- was a wee bit tricky, had a few hiccups along the way:
I kept on using: Enter pkgutil --expand ~/Desktop/
macOSUpd10.13Update.pkg ~/Desktop/Expanded wasn't working until I realized it wasn't using the proper name which was :
pkgutil --expand ~/Desktop/
macOSUpd10.13.1.pkg ~/Desktop/Expanded
-I had to use BBEedit (Text Wranger won't run int High Sierra the website said)
- for some reason I was unable to delete .xml from the file name.. until I Got Info and changed it in there
- in terminal, I then wasn't sure if it worked the first time, but also used the capital D so it
said chmod +x ~/Desktop/Distribution.xml to match the exact file name.
I'm going to restart now and see if the system still works! (maybe the installer did most of it's thing)