Usually, one accidental lens change with the camera powered on does not cause damage. The main risk is not immediate breakage, but increased chance of dust being attracted to the sensor or sensor cover glass while the camera is powered.

There can also be electrical/mechanical communication between camera and lens, so powering off first is the safer, intended way to disconnect them. On some cameras this may matter more than on others, which is why the manual’s advice is the best one to follow.

What to check:

• Mount the lens and confirm autofocus, aperture control, and general operation work normally.
• Check for sensor dust: photograph a plain bright surface (blue sky or evenly lit wall) at a small aperture such as f/22, then inspect the image at 100% for dark specks.

If everything works and you don’t see unusual spots, you’re probably fine. Going forward, turn the camera off before changing lenses to reduce dust and avoid interrupting any lens-camera communication.