So, we know that Guitar Hero 5 has—for a price—backward compatibility with existing song catalogues. But does it enable frontward compatibility in previous titles?
Answer: No. The DLC you'll get in Guitar Hero 5 is not usable in Guitar Hero: World Tour, according to Destructoid, and it's not a big surprise why. They're two different games.
An Activision rep told D'toid that "advancements and innovations" in Guitar Hero 5 make playing its DLC on previous versions a no-go. These features include vocal star power and and advanced drumming mode.
Bringing your existing DLC up to Guitar Hero 5 is possible through an export/import mechanism, but again, for a price, which Activision has not revealed. Also, it's applicable only to 152 of the 158 songs in the World Tour and Smash Hits catalogue, and no one knows which songs are the blacklisted six.