If you’ve deleted the cache and still have working upgrades, then it would seem your process of adding harmonies directly to songs actually worked after all, at least for some songs. Beyond that, all I can say is I’ve tested all the upgrades added since I created this fork of the project, but it’s always possible something broken snuck in. That’s why it would be helpful to have a list of the 2010 songs you’ve tried that are broken. If it’s easier, you could post the the upgrade dummy file somewhere and I could use that for reference.
You are about to leave Rhythm Gaming World and access the external songs DB. While no song or file sold for commercially available videogames is hosted there and only fan generated content is allowed, Rhythm Gaming World has no direct control over the database and no files are hosted here. Alright, we had to made you aware of this, now you can visit the database!