I was easily able to upgrade my Original LTE Shield tablet (only using wifi) to 7.0 from Optimized Stock 4.4.0. using the already installed TWRP 3.0.2 recovery (been using since MM install).
I downloaded 7.0 thanks to Bucklew's post here:
Looks like that Sero 8 is under whelming. I was excited when I first saw the specs for the new quad core Rockchip. But when I saw the underperforming Mali 400 graphics, I decided to pass. The Sero 8 even has the slower quad core "T"
You can still recover the Prime without backups. There are un-BLOB'D stock roms you can use to recover your tablet via APX mode. You might want to try that before replacing the motherboard.