Hello, and congratulations on your new milestone!
I upgraded from the AUR, which seems to have effectively deleted the MC27 install and left MC28 in its place. I started the new install, which had a clean profile. It seems the new install backed up my old data: The home/user/.jriver folder now had subdirectories of MC27 and MC28. I tried restoring my old library and settings from a backup, but the clean profile remained. Then I emptied the subdirectory of MC28 in home/.jriver and replaced the contents with that of of MC27, and my library and settings are back. Everything seems to be in order now.
Are you recommending starting a new profile?
Edit:
I just realized that when I went to import my old library and settings backup, the restore wizard had defaulted to /home/user/Documents/JRiver/Media Center 28, rather than /home/user/Documents/JRiver/Media Center 27. So in effect, I restored what was essentially a backup of a new, default profile from MC28. I needed to navigate to the Media Center 27 backup folder, which I just now did and was able to restore my library in the correct manner.