I'd like to be able to illustrate this question with a video or image, but they're copyrighted, so I'll have to explain as best I can!
Think of the famous "exploding hospital" sequence from the movie The Dark Knight. There's a great shot of Heath Ledger as the Joker walking away from the hospital as it starts to explode behind him. Also behind him is a sign saying "EMERGENCY", that has debris from the explosions falling across it.
Say I want to replace the "EMERGENCY" sign with something else, but to sell the shot I obviously still need to keep the debris falling in front the new sign rather than disappearing behind it. What would be the best way of achieving that?
My first thought is that trying to mask/key the original debris back over the top is a non-starter because it's all so tiny and there's so much of it, so should I think in terms of using particles to create my own debris to add over the top? Or is there actually a technique for utilising the original debris?