I was trying to use the N2 discs though (I hadn't got my replacement N3 discs yet)... And it wouldn't recognize the brain... Do I need to upgrade the N2 HU to N3/N4 software first, then swap the newer brain in?

Thanks!

OK, apparently you need to update the software to the new version first...

I put the N2 brain in, upgraded to N3 software, then hooked up the N3 brain...

Everything works now!

would this work with the N1 H/U?