With my SLR film camera, can I change the lens even with a film inside?

**************************************

Yes. The film will not be exposed until the button is pressed. When the button is pressed, a small mirror is lifted and the shutter opens exposing the current frame of film. This is several small steps to expose the film. But changing lenses does not expose anything.

*************************************

Yes. The camera has a curtain shutter at the film plane, not in leaf shutter in the lens, so when you take the lens off, the shutter, which is inside the camera body, is still closed.

Sure - the shutter remains closed, except when you push the shutter release. Many photogs carry their bodies without a lens (they put a cap on the opening) so they can slap the right lens on in a hurry

yes, u can do this
ever photographer do it whn they need a lens change while shooting on the same film

Yes. The camera body has a mirror that is normally in the down position. You look through the view finder and through a couple mirrors that allow you to see through the lens. When you press the shutter button, that mirror lifts to expose the film. When you take the lens off, that mirror is blocking the light from hitting the film.

Yes, just advance the film afterwards to get past the exposed film.