Speaking to GamesMaster, Harmonix's project manager Daniel Sussman has been outlining Rockband 4's commitment to ensuring that any old DLC you've bought will still work with the new game. The studio's already promised to make sure the old Rock Band instruments work with the new game, but that's just a couple of peripherals. DLC-wise, you're talking about roughly 2000 songs.
"One of the biggest challenges has to do with scale – we have so many songs. In fact, it took us over five years to build our library. We’re trying to redo all that work in a matter of months," explains Sussman. “Supporting DLC entitlements is a core pillar of the Rock Band franchise,” he says though. “We’ve always worked our butts off to respect the investments that people have made in our game. So, when it came time to roll up our sleeves and tackle the job of bringing seventh gen content over to eighth gen consoles, we knew we were in for the long haul.”
Then there's the small matter of more DLC to add onto that. “We’re still working through the details of our DLC delivery schedule. This is an area where we want to be open to input. I have my own ideas about how much content we should deliver, but it’s really meaningful to hear directly from our community”, he explains.