Ideally, this is not possible. You have to delete the video from the older channel and upload it to another. If you don't want to kill the existing link, follow these steps-
1. Upload the video to new channel.
2. Use YouTube annotation to black out current video, by adding a text like "This video has been moved here" and link this annotation to the new video.
However, this has some disadvantages-
1. Please note that this is not an ideal solution to your issue, but a work around.
2. You will lose the YouTube ranking of your video.
3. You will lose comments and views.
There is absolutely no other way than this as it is NOT allowed by YouTube to transfer videos between channels. This is officially confirmed by YouTube.