I tried to replace a video but after hours I still see no progress.
If I click on it in the video list, I see the message "(2/3) preparing formats". In the "Video Formats" tab, I see the following message "Video replacement is currently being processed".
Video replacement can take some time. If the status of the video replacement remains unchanged for a long time (for example, two hours or more), it is quite likely that the video replacement is stuck.
The reason for a stuck replacement, like that of a broken upload of a video, is that you may have been logged out during the replacement process.
You can restore the previous version of the video by clicking on the arrow on the top right-hand side of the video item. After restoring the video, the original video formats are displayed. Now you can start the replacement again.
The replacement restore feature is documented here.