I’d recommend using DVD Rebuilder. It has a built in option that does just this. Select it and in a couple clicks you can re-encoded and rebuild the entire disc without any effort.
http://www.jdobbs.com/
It is under the Options Tab: AVS Options/AVS Expert Options/Convert from LB 4:3 to 16:9.
Edit: I do prefer to do this while converting PAL to NTSC. PAL has more lines so you do gain some picture.
Otherwise, it’s usually easier to select the Zoom picture mode on the TV.