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]