I believe the case study you are referring to is the one dealing with the existing building located in Brooklyn. If this is the case, check out the reference document “NYC Landmarks” and scroll to page 46. This is Chapter 3, Section 3-02 Repairs, Rehabilitation and Restoration. The first item states “(a) Repairs. Deteriorated windows can often be repaired and made sound and fully operational. A permit is not required to undertake ordinary repairs including…” and then lists things that are included to be changed without a permit. Cracked glass, damaged sills, and the lack of weather-stripping fall under one of these categories.
Hope that helps! Be well!