As mentioned, the only real problem with cameras and humidity/fogging come from the transition to air conditioned, cooler, drier air into hot, humid, moist air, and back again. Once the camera and lenses have acclimatized to the conditions you're going to be shooting in (it's generally a good idea to prepare for this a good hour or so before you need to start shooting, by turning off the AC, putting the camera and lenses on a patio or balcony or open windowsill if secure, and letting it adjust to the outside temperature and humidity)...it doesn't really matter if you change lenses in the field...everything's equalized by then. If you didn't properly acclimatize a lens - say one you left closed in a bag in the room before you left and when it comes out of the bag it fogs up - then you may have issues - but even then, most cameras can stand a bit of fogging and defogging without any real harm - unless it's something happening daily for months on end, where that condensation can start to cause corrosion to parts inside the lens or body, or develop mold if the lenses aren't dried out when storing with some silicon or other similar methods.
As for any dangers changing lenses in the field - just simple precautions such as always powering down the camera when changing lenses (it's not a necessity, but cameras that are powered up can sometimes have enough of a static field generated to attract small particles to the sensor), always keeping the lens opening pointed down when swapping lenses, and minimizing the amount of time that lenses and camera are open and uncovered during the swap, should allow you to make all the changes you want or need without any issues. Short of changing lenses in a downpour, or a sandstorm, or covered in floating pollen...brief lens changes with the camera pointed down can be executed many times a day in the field without sucking too much unwanted stuff into the camera body.
