Changing the aspect ratio of a video
It's easy to do this with FFmpeg which is run from the command line.

Method 1 – this will size the video appropriately and encode the result in H.264 MP4 format:

ffmpeg.exe -i inputfile.mpg -vcodec h264 -s 720x576 -aspect 5:4 outputfile.mp4


Method 2 – this will set a hint in the file as to the size it should be displayed at (with no recompressing or quality loss). VLC should respect this although not all players/editors will:

ffmpeg.exe -i inputfile.mpg -vcodec copy -aspect 5:4 -acodec copy outputfile.mpg


Method 3 – if you detect a decrease in the quality of the MPEG you can compress it at a high-bitrate MPEG-2:

ffmpeg.exe -i inputfile.mpg -vcodec mpeg2video -aspect 5:4 -acodec pcm_s16le -b 30000k outputfile.mpg


Good luck!