If you want keyboard and vocal harmony parts added to your old Rock Band songs then you're going to have to buy them again.
Specifically, you'll need the Rock Band 3 version of the song charged at, presumably, the default DLC price - the Harmonix website doesn't clarify.
The only upgrade available for old Rock Band songs is to add the Pro Guitar and Bass tracks, a procedure that costs 80 Microsoft Points/Ł0.59/100 Wii points.
Today marks the arrival of Bon Jovi songs to Rock Band 3. Existing Rock Band songs "Livin' on a Prayer", "You Give Love a Bad Name" and "Wanted Dead or Alive" have all been updated to Rock Band 3 versions.
Each song costs roughly a quid, or it's a tenner for the full 12-song list.
Rock Band 3 may have scored 10/10 on Eurogamer when released late last month, but the game is struggling to find a audience, as low first-week sales gave way to even lower second week sales. Mind you, the Pro-style RB3 instruments are still to arrive in many shops - perhaps, like Carl Lewis, Rock Band 3 is a late starter.