The reasoning is consistency. We offered a rebuild last time because we switched from the Story Origin mechanic to the PHB+1 mechanic and felt some people had chosen ToD, or even PotA, because that was the season they were playing and not necessarily because it had any mechanical effect on their builds. In that situation it seemed only fair to not have people be stuck with ToD as their +1 since it had nothing in it; so we gave a rebuild. We did not give a rebuild because the SCAG was being released. As we like consistency, you should expect NO rebuilds when new books come out.