Can I change Lenses on my 35mm camera while the film is loaded, or will it expose the already loaded film?

Yes you can. The front with the lense is closed off from the back with the film by the shutter so no light will get back there.

Professionals do those lens changes very frequently when they are on road trips with more lenses than cameras.

If it were me I would do the lens changing away from bright sunlight just to be extra cautious in case there is any kind of abnormal light leak around the shutter. But even without that precaution you should be fine.

As long as you don't have the shutter open you can't expose the film that way.

Yes you may without damaging the film

yes the shutter is closed so the film will not be exposes unless you push the shutter button

it depends on the camera-on most slrs the shutter is in the camera body and protects the film. in some older ones with screw mount lenses and some older range finders the camera uses a leaf shutter system (the shutters in the lens) and with these cameras popping the lens of while the cameras loaded will trash your whole roll of film.

yes you can change lenses and you wont get any exposure if you face the camera downwards away from the direct sunlight. it may fog slightly but not much unless you set off the shutter.

I had a leaf shutter system SLR with interchangeable lenses. You don't damage the film because the mirror is down always, except for the moment the shutter is released. If this were not so, your film would becomed fogged because of light coming in the lens normally. I never heard of a rangefinder camera with interchangeable lenses, that would be one crazy focus system.