Change the Aspect Ratio to remove the black bars (iPad)

Hi!I just discovered this forum looking for a way to convert AC3 to AAC and I successfully accomplished that task.:)

What I am looking to do now,if you guys can help me,is to change the Aspect Ratio (AR) of an MP4 container (H264.AAC,SRT 16:9 720P) to 4:3 AR so that I can play the video file without the black bars (letterboxing).

I tried getting some APPS that are able to stretch the movie to fill the screen but when I use those APPS and stream content VIA UPNP Media Server(s),the playback is choppy,laggy and in some scenes the image is pixelated and turn greenish.

When I use the default Media Player that is implemented in the iOS (QuickTime I think) or the Ipod Plugin found in those APPS settings,the movie plays smoothly,perfect.The only downside is that I'm not able to change the AR using the Plugin.

So what I'm looking for is to play video files on the whole screen of the iPad (4:3 AR).

I also have a question regarding the AR.If you change the AR of a video file(16:9 to 4:3),can this affect the quality?

I know that this ISN'T an iPad forum but I thought you guys can help me with a recommendation of a program or with a HOW TO FOR DUMMIES tutorial.

Thank you in advance!:)