My suspicion given past exams is yes, there will be an upgrade exam with a subset of the questions offered on the new exam. If you already have the SCBCD, I'd wait to see what they're going to do (since the upgrade is cheaper).
Insofar as the exams themselves, I've taken upgrades previously from Sun (SCWCD and SCJP) and would admit that I'm torn which way to go - certainly, saving the money is nice, but I think that at times, I'd also like the safety-blanket of the easier questions that are omitted when you're only being questioned on changed/new features...
Theodore Jonathan Casser
SCJP/SCSNI/SCBCD/SCWCD/SCDJWS/SCMAD/SCEA/MCTS/MCPD... and so many more letters than you can shake a stick at!