Perform a multipart/form-data POST. When a replace is performed, then the preview associated with the photo or video will be regenerated if the content is a photo. On the other hand, if the content is a video, then you should ensure that no preview will exist after the replace until the user adds a thumbnail preview to the video.
You can derive the URL from the video document entry. Use the web address specified as the "href" attribute of the http://www.ibm.com/xmlns/prod/sn/cmis/multipart-form/object"> element.