It's a little tricky, involves some pinpoint soldering. Wayne's got some Q3L emitters you can buy from his site, I believe (well, if he hasn't run out yet)... You'll have to cut the 2 pos/neg tabs on the LS of the BB500, and do a simple swap and resolder the Q3L's contacts back in... You'll have to pry off the LS first, and you'll need some thermal epoxy (like Arctic Silver/Alumina) to apply to the bottom of the Q3L for better heat transfer to the sandwich base...
From the looks of it, the trickiest part would be to connect the pos/neg tabs to the base of the sandwich...