4 answers
Just adding to @Judi Sohn's comments, make sure you understand that NPSP v3 has a different account model: https://powerofus.force.com/articles/Resource/NPSP-What-is-an-Account-Model This is the biggest stumbling block for transferring to the NPSP v3. And here's a user group specifically for this topic:
https://powerofus.force.com/_ui/core/chatter/groups/GroupProfilePage?g=0F980000000D5pE
