I've tried two methods so far and neither have worked:
1. Making a backup with MC 15 and restoring in MC16 -> MC16 refuses to accept MC15's backup
The "restore settings" option causes it to fail. Otherwise it works fine.
2. Installing MC16 Portable Install into MC15 Portable Install directory -> MC16 does not pick up MC15's database
There is no way to know if a portable MC exists somewhere in the file system. I guess that normally MC checks the Windows registry, but since the install is portable there is no sign of it in the registry. MC would need to use brute force and check each and every accessible folder.
OK I've had some success by renaming the folder 'Media Center 15' in %mediacenterfolder%\Application Data\J River
to 'Media Center 16'.
I'm not sure if this is likely to cause issues however.
The old ini files contain some references that point to the install path. You may want to fix them if you use a different folder for the portable MC16. Though I don't know if the stored install paths are used for anything. You cannot uninstall a portable instance by running the uninstaller.