Air Carrier Captain reported a fume event during passenger boarding. After troubleshooting and conferring with flight attendants a logbook entry was submitted which ultimately resulted in an aircraft swap.

Narrative

During boarding; our inflight notified me that they smelled a strong unpleasant odor of burning metal or burning plastic in the aft galley. When I walked back there; I could smell the same thing. I immediately turned off the APU bleed and we notified ZZZZ Maintenance. While entering the issue in the logbook; I could smell a faint odor similar to acetone or other chemical in the flight deck. The FO (First Officer) described the odor as a crayon smell. These descriptions are characteristic of fumes events as a result of oil burning in the ECS. Two inflight crew members experienced mild nausea and I experienced a mild headache which resolved about 30 minutes later.The first Mechanic to arrive told us that the plane would be taken out of service for several hours and that we could expect a tail swap. We communicated with operations to deplane all customers and did so.The next Mechanic to show appeared to be in a managerial or supervisory role and was dressed differently than the others. He very sternly questioned me as to how I knew it to be a fume event if all packs were off. I explained that I turned them off to stop the fume event from occurring. He then told me that he didn't smell anything and that it could not be a real fume event because the inbound crew had no problems last night. I explained that 5 crew members confirmed the odor along with 2 customers who complained about the odor. He then explained that he would run both packs once fully deplaned and that he would simply sign it off and return it to service if he doesn't smell anything. I professionally advised against it; and recommended that he appropriately follow the guidance in the Aircraft Maintenance Manual and Information Notice. I filled out the Maintenance Odor Form and handed it over to him.Shortly thereafter; we were given a different aircraft and flew uneventfully to ZZZ. Upon arriving in ZZZ; multiple customers thanked us for changing aircraft because the first aircraft; 'was very smelly' (their words). An elderly woman and her caretaker said the smell was awful in the last row.