How will Rock Band 3's new instruments work on existing songs?

Solution 1:

Currently:

What we know:

• Most, if not all, of the legacy catalogue of RB(meaning all the DLC along with the past games) has been charted with the cymbals one uses with the pro drums. If you look at the animations of the drummer in-game, you'll notice that the drummer will play on the cymbals or on the toms on notes of the same color. This is not conjecture based off of that, however; HMX reps confirmed this.
• Everything new with the keyboard and guitar controllers will only be available for RB3 and all forthcoming DLC(from RB3). Same source.
• "There are a lot of cues and “hooks” in [HMX's] authoring flow that have always been there; [they] just haven’t had the software cycles to take advantage of them". Again, same source.
• Harmonix will be re-releasing some old DLC with full support for RB3's new features. Currently they are asking for the community to chime in on which songs they would like to see re-released first.

What we don't know:

• What exactly the cues and "hooks" are aside from the drums. I have, however, noticed in-game that there are times in RB2 when the guitarist and the lead singer will sing together, implying that some of the songs in their legacy catalogue may have support for vocal harmonies in RB3. No concrete evidence, though.

Updated August 16th: HMX will be re-releasing old DLC with full support.

Solution 2:

Harmonix stated that the existing drum part encoding already differentiates between toms and cymbals - so most of the existing songlist will be essentially "Pro Enabled."