Okay, my idea above won't work.
In YouTube there are two places you can edit videos.
One is "Enhancements" where you can trim the video, stabilize, change fill light, other visual effects and add audio. When you save it, the changes are saved to the ORIGINAL video.
The other way to edit is using "Video Editor." The only way to get there is to click "Video Manager" and then the "Video Editor" tab. In the Video Editor you can combine 2 or more of your previously uploaded videos into 1 video but Video Editor always creates a NEW video.
You can't add a video to a previous video (or replace the previous video with another video) and then save it to the old video. When you combine 2 or more videos, the result is always a NEW video. (Bummer.)
BTW, the video mentioned above that previously ranked well but fell out of the SERPs after I added additional Tags, is back on page one of the "Everything" SERPs. It's actually in a bit higher position, it was third of the three videos shown but now it's the second of the three videos shown in the #9 spot on page 1.