No regrets

Luffy and Sabo were about to pay their respects to Ace and suddenly the Pirate King Monkey D. Luffy finds himself back at the Sunny... days prior to the crew's second encounter with Bartholomew Kuma. What does this mean? Why has he gone back? And why does he has the memories of those twelve years afterwards? Time-skip verse. No pairings. AU. Nakamaship.