Green Day: Rock Band owners looking to keep those tunes for future use need to export those tracks to Rock Band 3 by April 30, Harmonix announced.
The Green Day: Rack Band–to–Rock Band 3 transfer needs to be completed before April 30. After that, the code will be a useless string of numbers and the Green Day tunes will need to be repurchased as DLC.
To export, go to “extras” from Green Day: Rock Band’s main menu and plug in the 12-digit code included on the export insert in the game case. According to the Harmonix support page, there’s a $10 export fee to pay as well. Though, this is no doubt considerably cheaper than purchasing Green Day DLC albums, packs, and singles to recreate set lists (which is the only option available after April 30).
Existing Rock Band DLC will be transferrable to Rock Band 4 when the PS4 and Xbox One rebirth of Harmonix’s much-beloved plastic instrument rhythm game launches later this year.
Source: Harmonix forums