Katara knows in her heart that things are different between her and Aang now. He is no longer her boyfriend, but the man that she sees spending the rest of her life with, and she's ready to show it.
Katara, Zuko, and Iroh find themselves in the exact places they were when Aang came back into the world (season 1 episode 1). The only thing they can remember before waking up there is the moon festival three years after the end of the series. How did they get back to where they were three years ago? Why did Yue send them there? How can they change what happens and not affect their present?