A320 flight crew reported a fumes event during pre-flight described as a dirty sock odor. Captain requested Maintenance which resulted in an aircraft swap.

Narrative

Upon boarding the aircraft; ground air was being used with the APU shut down. As it was 90 degrees in the Cabin and the ground air was putting out 100 degree air; per the COND System page; I started the APU and went outside to conduct the preflight and request the ground air to be disconnected. PACK 2 was already on MEL and selected off. The First Officer turned the APU Bleed on once the ground air was turned off. During the preflight; I noticed oil on the underside of the tail; forward of and around the APU inlet flap. Upon entering the cockpit a few minutes later; I was immediately aware of a strong 'dirty sock' odor. The First Officer said it smelled 'musty.' I immediately turned the APU Bleed off and called Operations to ask Maintenance to come to the aircraft. The Maintenance Technician briefly turned on the APU Bleed and then quickly turned it back off after also detecting the 'dirty sock' odor. There were no inflight crew or customers on-board during this time because the inflight crew was still deplaning at a different Gate. Upon further review of the logbook; I read that the APU had had a shutdown event and oil contamination in the same area around the inlet flap a few flights earlier. Operations decided on a tail swap for our flight.