I would have to disagree,
I've managed to get almost all my dlc for rock band / rock band 2 to work in both games , (even from usb drive). If you unlocked (yaris swap) your rb songs, and place them in the correct folder , they will work no problem.
I've done this with single songs , 3 songs packs , they all worked. Only my full album of Pearl Jam Ten doesn't work.
I did not have to get the game updates.
The rb1 game updates , to allow to transfer songs from rb1 to rb2 , will only work if you run a cd , and not if you run from usb drive (this game has been updated to an incompatible version) is what you get from trying the update when the game is on usb drive.
If you yaris-swap the exporter license , then boot rb1 , it will allow you to transfer all songs to hdd , and will then work in both rb1 and rb2