Priced at $14.99, the export will be available within the same console family for people that are verified purchasers of Rock Band 3 only on PlayStation 3 or Xbox 360. Harmonix explained:
We will be automatically verifying your purchase electronically, which means that you’ll need to have bought a new copy and played Rock Band 3 for our export process to work. If, for some reason, our electronic check doesn’t work for you, please contact our Customer Support portal here.
While the Rock Band 3 game is expiring [on October 30 on Xbox Live; it’s disc only on PS3], the export offer itself will be valid for the foreseeable future. As long as you have the game, you can take your time and we’ll give you plenty of notice before the export offer is set to expire.
When the Rock Band 3 export is available, any tracks from the disc with authored guitar solos will feature upgraded Freestyle Guitar Solo and Freestyle Vocal features. You can also toggle between the new Rock Band 4 features and legacy features, like those guitar solos, in the middle of each track.
To help answer any questions you might have about exporting old Rock Band games into Rock Band 4, Harmonix created this infographic: