Swapping out a video file for an upgraded version of a video on youtube is referred to as an Inline replacement.
Criteria
Inline replacements require a minimum of 5000 views, and can only be done when replacing videos with higher quality (resolution, aspect ratio) audio or video.
Videos will not qualify for a replacement if you are requesting an editorial change, such as removing scenes or intros.
Process
Submit your request using the Inline Replacement Request Form: https://ops.vevo.com/inline/
FAQ
How long will it take for my video to be replaced?
Requests are processed with YouTube at the beginning of each month for all approved requests submitted during the last month. For instance, a request submitted on January 15th will be processed the first week of February.
If there is a reason for your inline replacement to be rushed, please include a justification in your replacement reason.