16-bit is only applicable to a limited no. of image file format, eg. TIFF, PNG, PSD... etc. If your image file is in JPG format, it will only be opened or saved in 8-bit format. If your original file format is 16-bit, you should carry out all your post-processing in 16-bit mode before converting down to 8-bit format for saving as there would be little or no loss of image quality during post-processing in 16-bit mode unlike the case with 8-bit post-processing.
If your original file format is 8-bit eg. JPG, there is no point in changing it to 16-bit mode as all the info that was lost cannot be retrieved by such a conversion. And there is a huge amount of info that's dumped when converting from 16 to 8-bit. For example, the file size of a typical 3,072 x 2,048 16-bit TIFF image is easily more than 30M; when converted to 8-bit JPG, it is only slightly more than 1M !