Does the HDC bylaw require all structures to be of early 19th century architecture?

No. The bylaw is a preservation document which means that current architectural styles of the buildings in the district should be preserved. The bylaw preserves all the changes that have been made to the buildings over time. This preserves the entire history of the building - not just its initial design. The HDC approves changes which it thinks are sympathetic to this mix of styles and periods.