Owners of either Rock Band or Rock Band 2 will not be able to download any future releases on the title's in-game app store.
Effective as of October 26th 2010, all DLC releases will work exclusively with Rock Band 3 only. Owners of the new game will be able to download tracks from the store's back catalogue.
Speaking to Joystiq, a representative for developer Harmonix claimed that future DLC won't work with the old games due to the new authoring standards, inclusion of new instruments, new lighting / venue cues, etc”.
Owners of either of the older games who log into the titles' in-game store from this point on will not be presented with any new releases, though current releases will remain available.