To convert 16:9 to 9:16 without cropping, pad your video to vertical. If you must fill the frame, crop after scaling. Our converter shows both approaches.
Pad example (copy)
ffmpeg -i in.mp4 -vf "scale=1080:-2:force_original_aspect_ratio=decrease,pad=1080:1920:(ow-iw)/2:(oh-ih)/2:black" -c:a copy out_pad.mp4
Crop example (copy)
ffmpeg -i in.mp4 -vf "scale=1080:-2:force_original_aspect_ratio=increase,crop=1080:1920" -c:a copy out_crop.mp4