Higher resolution videos take more time to render depending upon the upload traffic and other factors. In this case, we'd suggest you to give it sometime for the system to update the quality of the video. Let us know if it's still otherwise.
Appreciate the info. Please send a feedback report directly from YouTube with these steps: https://goo.gl/WdI0BK . While every individual report may not get a direct response back, we do review everything and use the details to investigate and fix issues. Thanks!