Air Carrier Captain reported a fume event during preflight duties at gate. Captain contacted Maintenance and Dispatch which resulted in an aircraft swap.

Narrative

Arrived to the aircraft 55 minutes prior to departure. Walking into the forward galley; I immediately smelled an unidentified odor. Walked into the Flight Deck and was overwhelmed by odor. I initially thought it was a dirty sock smell and assumed it had came from packs. However; aircraft was on ground power with APU and APU BLEED off. My secondary thought was wondering if one of the chillers was leaking refrigerant. Said hello to two of the flight attendants (in first class) and asked if they smelled the odor; they both did. Asked them to come to the flight deck and they both had a strong reaction to the smell. One said she thought it smelled like a dead animal. 43 minutes prior to departure; I called ZZZ Maintenance and described the issue and asked for a Mechanic. Meanwhile; one of the Flight Attendants declared she was developing a headache. I wondered if I was; also; developing the same symptom? I brought all of the Flight Attendants; as well as; the First Officer into the conversation. At the conclusion of the conversation; and in the interest of safety; I communicated with the Gate Agent to hold off on boarding the aircraft. Maintenance told me I needed to write it in the AML but advised me the check would take a couple of hours. I contacted Dispatch and he advised me we had another aircraft across the hall in the other ZZZ terminal. Crew vacated original aircraft; waited on new aircraft to deplane and expedited the boarding process. We got off the Gate 25 minutes late from original departure time and made up some time in flight the best we could.Not knowing what the odor was and feeling like there was a possible correlation between the odor and mentions of headaches made the aircraft a safety concern.