Change it in your DVD player menu, if you do everthing is going to have the incorrect aspect. Balls will take the shape of eggs. You don't change the resolution, 16:9 and 4:3 use the same resolution, you cahange the aspect ratio flag. If it's a 16:9 video and you search through the ifo file you'll find instances where the tracks are labled 16:9.
Don't use ifoedit so I can't tell you exactly what you need to do. I know you can though.