Problem: I have two directories that are similar in structure but different in content. Say one is an older snapshot of the other. I want to copy the stuff from the new directory into the old directory (recursively).

Copy:
rsync -a SOURCE/ DEST/ --ignore-existing --whole-file

Move (no clobber):
rsync -a SOURCE/ DEST/ --remove-sent-files --ignore-existing --whole-file

Move (and clobber):
rsync -a SOURCE/ DEST/ --remove-sent-files --whole-file

Move, but only update with newer:
rsync -a SOURCE/ DEST/ --remove-sent-files --update --whole-file