Two things for anybody that finds this in a search.
First, when Julia says restart the sever, she means literally the server OS. I was unable to restart the service in Windows Server 2012. This needs to be taken into account when planning for failure.
Second, if you follow her steps, when the server restarts you get a messaged that the TeamCity installation is corrupted.
Where Build_X corresponds to the new version you attempted to update to(in my case it was BUILD_51228 for 17.4.4)
In addition to copying files over from the .old directory, you also have to delete the BUILD_X files from the “TeamCity home” and “TeamCity home\webapps\ROOT” directory.
Only then was I able to restart the server and recover my old installation.
No I have all of my old installation files, interspersed with thousands of new files from the failed “automatic update”. I am not feeling confident that I will ever be able to successfully perform an automatic update. If I was a betting man, I am going to have perform a clean install…