B737-800 Captain reported a fume event during gate pushback resulting in a tow back to the gate for passenger deplaning. Maintenance assigned an aircraft swap and flight continued with a normal departure.

Narrative

During pushback and prior to engine start; an unusually strong burning odor was detected on the flight deck and confirmed to be present in the forward galley. Passengers in the forward section of the aircraft also mentioned the strong odor. An elderly lady seated in the front row began to cough and complain about the strong odor. I confirmed with the Flight Attendants that no smoke was present. I elected to have the aircraft towed back to the gate and deplane the aircraft to include all passengers; crew personnel & belongings. Maintenance was called to troubleshoot and they suspected the air conditioning ducts had residual contaminants that burned off when the packs were first brought on line. I verified the odor was isolated to the forward section of the aircraft by conducting a sweep of the middle and aft sections as part of the terminating procedure. An aircraft swap was arranged and we continued the flight to ZZZ1.Many times I have experienced a faint odor during an originating flight on a cold originating aircraft. This happens when the automatic temperature control drives the mixing valves to a very warm setting to deliver hot air into the cabin until the selected temperature is reached. The odor today was much stronger than I have experienced under these conditions; which led to the decision to gate return and deplane passengers and crew.