The one thing which is missing from the "Appleinsider "How to downgrade from High Sierra to Sierra" is this. I have performed this suggestion with the following results:
1. Backed up my MacBook Pro using Time Machine.
2. I followed the steps outlined in this article.
3. I rebooted my Mac with the Flash drive containing Sierra.
4. I used Time Machine to restore my Data
5. when I rebooted my Mac, expecting it to just copy the data, High Sierra was reinstalled along with all my data.
The post doesn't work. Perhaps if you used a different backup program, it might work, but using Time Machine restores everything, including High Sierra. Sierra gets overwritten and you're back to the same place you started.
I would suggest the "Appleinsider" look for other methods. This one doesn't work. BTW: I'm using a 2012 MacBook Pro running High Sierra 10.13.6. On a 2.3 GHZ,Intel Core i7
Let me know if you've experienced the same problem. I also have a problem with High Sierra staying connected to my router. I'm using a Windows PC (UGH) to post this because High Sierra WiFi keeps telling me "Looking for Networks" then WiFi=On, then back to looking for networks until it finally gives up and drops WiFi all together. I'm sitting five feet from the router and still getting these messages. I've performed a hardware test by (Pressing "D") while booting up. No problems were found. So, I know my machine is working correctly. My wife has the identical machine, but she's running El Capitan with no problems. If I have to I'll drop all the back to this. It doesn't make sense, unless you look at Apple trying to get you to upgrade your MacBook Pro (with no CD-ROM). lol