When the thermal boundary is the attic floor, then it is modeled on the Attic page. Model this surface for the base building, but set the improved Area to zero.
When the thermal boundary is the roof deck, then it is modeled on the Ceilings (Vault/Flat) page. Model this surface for the improved building, but set the base side area to zero.
Both the Attic and the Ceiling (Vault/Flat) measures must be selected for the conversion to be effective. Total cost and savings of moving the boundary can be seen by summing the two measures.