Long, long ago, YouTube was 4:3 only. Then the age of widescreen came and so they changed everything to 16:9. This left many videos in a square peg oblong hole situation (as in the video just doesn't fit the player window properly). So here are a few ways that you can change the aspect ratio on your YouTube videos and NOT have to re-upload.
Back in late 2008 YouTube switched all players to widescreen. That means that your 4:3 videos are smaller in the player window and have black bars on the sides. This is sort of the opposite of what happens when you watch a widescreen video on a standard television where the bars are top/bottom.
If you have a lot of 4:3 (standard TV-sized) videos and you want to display them without the black bars on the sides or some other way, YouTube gives you some options. You can basically either crop or stretch to get it to fit into the player how you want. This will also work for videos that have been stretched improperly or where the aspect ratio is not what YouTube thinks it is.
So to get around these problems, YouTube has implemented some options for uploaders that will help you get the most out of the space provided via some quick additions to tags.
To use these just drop them into the tags section of the video in question and they will be displayed as requested automatically.
How To Fix YouTube - CROP:
There's a single option available for you to crop your video so that the player pretty much just zooms in on the video so that you have a 16:9 version. This will of course cut off any of the material that falls outside of that (basically the top and bottom). It will also zoom in if you have widescreen content that is being shrunk for some reason (like YouTube thinks it's 4:3). This might not be an ideal option, it depends on the original content. You can test it out by using: yt:crop=16:9
How To Fix YouTube STRETCH:
There are two ways that you can stretch the video to get it to fit. You might need this is your content looks squashed because it was widescreen but is showing in 4:3. If used on 4:3 content it may distort the video content and perhaps affect the quality of it as well. Maybe not ideal for some videos but might work well enough for some others. Be sure to check your video after you implement the tag. To use it put in the tag: yt:stretch=16:9
The second stretch option YouTube has provided is to take 720x480 content and scale it to 4:3 properly. This will fit it to the player window so that it fills it completely. This works best if say, the video looks stretched out already by adjusting the aspect ratio to what it should be. The tag for this is: yt:stretch=4:3
How To Fix YouTube Video QUALITY:
They have also provided a tag to have the video default to a high quality stream if available. This will give your viewers the best possible viewing experience automatically without having to click the icon. That is, provided that you supplied them with a good enough version of the video to reach that standard. For this the tag is:yt:quality=high
Here's a quick video that will show you how to get rid of the black borders on your videos (we didn't make it, just so you know):
Here's some examples on youtube with using the code: