Display Aspect Ratio

There is a difference between Sample Aspect Ratio (SAR) and Display Aspect Ratio (DAR).

Display Aspect Ratio (DAR) - a value in the header.

Sample Aspect Ratio (SAR) - count of pixels horizontal ratio to vertical.

To scale the image and change SAR (while transcoding), try:

ffmpeg -i <INPUT_FILE> -vf scale=720:540 -c:v <Video_Codec> <OUTPUT_FILE>

On the other hand, if you just want to change the metadata flag and adjust the DAR, you will be able to stream copy the video. To do this, try:

ffmpeg -i <INPUT_FILE> -aspect 720:540 -c copy [OUTPUT_FILE]