Since you are talking in the context of long exposure shots, there are two possible scenarios when Long Exposure Noise Reduction (LENR) is enabled:

1. The shutter is still open and recording the scene. In this case, if you turn off the camera the shutter will close immediately and the exposure in progress will be recorded. No LENR dark frame will be recorded. There will be a short delay before the camera is completely powered down, but it will be completely safe to go ahead and move the camera and remove the lens during this time.

2. The shutter is closed and Long Exposure Noise Reduction (LENR) is recording a dark frame. In this case, if you turn off the power during the recording of the dark frame, the camera will complete the dark frame and save the image before powering down. It is completely safe to go ahead and move the camera and remove the lens during the recording of the dark frame, as the shutter is closed.

If Long Exposure Noise Reduction is not enabled, turning the camera off when the shutter is open will result in the shutter closing immediately and the image being saved before the camera is completely powered down. It is safe to go ahead and move the camera during this time.

Please note: In all cases the EXIF data will reflect the selected shutter speed and LENR option selected at the beginning of the exposure, not the actual time the shutter was open or whether a dark frame was actually generated and applied to the photo.