Z.B.
If it's a Nook HD, it uses Nook Apps and Google Apps. You do haves to start from scratch, but you can should be able to get most of the same Apps. If it's a Nook Color or Nook Tablet you can only use Nook Apps. Either way, yes, you have to start from scratch.
Barnes and Noble does off a free app very Friday. Just type "Free Fridays" into the search box. The offer a free book and a free app. The free app is almost always kid friendly and very often educational.