I've had a Nook HD and Nook HD+ from ancient SDs sitting around for several years. Once and a while I plug them in with the chargers so they don't totally die. They were Clockworkmodded into Android 4.4.4 ages ago. I also recently got another stock Nook HD for some kind of future project for ten bucks.
This led me to research. I came across this.
The link gets you to a very clear Android 7 sd card conversion for any stock Nook HD+. The linkee gives you another set of files and instructions for a smaller Nook HD.
I created two bootable micro cards per the blog instructions and it didn't work on my two 4.4.4s.
Then I remembered booting into recovery using the N button and Power button trick.
I put the two real files (7 OS and GApps) in my 4.4.4s internal memory, and booted to recovery with the buttons. Wiped data and dalvik like in the old days. Then installed the two zip files in order. Lo and behold, two Android 7 tabs.
Then I tried the micro card boot on the ten buck stock HD. No go following the instructions. Then it occurred to me to try to boot into it using the same N and Power trick. It worked, booted into CWM from the card. Now I've got three former paperweights as Android 7 tabs where everything seems to work.