So I decided to hold my nose and jump in tonight.
I booted to the stock recovery and cleared the system cache. Then I enabled Opera VPN, switched region to Canada, and snagged the OOS 4 update and switched off the VPN during the download (to speed up the d/l process). From start to finish took between 10 and 15 minutes including optimizing 155 apps.
Interestingly, a couple of stock icons were missing (clock and contacts). But I located them in the apps and quickly put them back in place. I checked my APN settings and instead of 2 APNs, I had 4!
1) NEXTGENPHONE (nxtgenphone)
2) ATT PHONE (phone)
3) ATT WAP (wap.cingular)
4) ATT PHONE (phone)
I tested 1, 2, and 4 and can make calls, so it looks like my mobile data works fine. I think 2 and 4 are duplicates. I was using ATT PHONE on MM, but now am using NEXTGENPHONE on Nougat, but see no functional difference. I did not have to input any of my APN settings which is interesting, based on the experience of others where their settings were wiped.
After the installation and reboot, I simply had to choose my launcher (Nova), then went into settings and tweaked a few things (split screen on the recents button (long press), etc. I'll be a few days tweaking, most likely. I need to see what Messages is all about (vs Textra and Messaging).
But overall, a painless update from MM. I left my file system as EXT4 for now.
I'm happy to report this was totally a non-event.