Yes — on a typical film SLR, you can safely change lenses while film is loaded. Removing the lens does not expose the film because the film is protected inside the camera body by the shutter, which stays closed except during the actual exposure. The mirror and shutter assembly block light from reaching the film when you’re not taking a picture.

So switching from your 28mm to your 50mm halfway through a roll is normally fine.

As with any interchangeable-lens camera, it’s still wise to change lenses reasonably quickly and in a clean environment to reduce dust getting into the camera. There were some uncommon older designs that used different shutter arrangements, but for an ordinary film SLR this is not a problem.