Tsunayoshi Sawada was raised knowing the mafia but in Japan by his mother who doesn't want him to be involved and she kind of ignore this fact from both her husband and her son. His father's still absent being the head of CEDEF but they do communicate although Tsuna isn't tutored to be in the mafia as he's still Dame.

What changed is that Federico is the tenth after the death of Enrico and Massimo. Xanxus the head of Varia, so Reborn never met the potential Japanese candidate. 

Hayato Gokudera left his family at eight because he thought his father ordered his biological mother's death (Lavinia) and he's raised by Shamal and worked under various mafia family, including Vongola, without really being part. Sort of hitman like Reborn.

Takeshi Yamamoto befriended Tsuna because he saved him from killing himself (actually Tsuna attempted to be badass by pulling a stunt to catch him but being dame, he nearly let him slip and cause a ruckus and got bitten by Hibari-san for having break Nanimori's calm). Impressed by his move (totally accidental), he listened to his advice of doing kendo instead of baseball.

Lambo Bovino stayed in Italia but was sort of a wanted person for the possession of ten years bazooka. Being a spoiled brat, he didn't even notice.

Ryohei Sasagawa is in the box club and being a distracted big brother. 

Kyoya Hibari is Nanimori's protector, whether in Nanimori middle high or high school.

Nagi, a poor hospitalized child lost her internal organs and right eye. She got givers three years later after being miraculously saved by an illusionist.

Mukuro Rokudo was subjeted of experiments by the Estraneo Famiglia mafia till his ten years old, then after freeing himself and two others (Ken and Chikusa), they were taken by Lanchia's famiglia but he manipulated him and slaughtered them all. At fifteen, they travel to Japan and met Nagi (later renamed to Chrome) after being imprisoned somewhere between France and Switzerland (where Birds, Bloody twin and M.M got into the gang after breaking out of prison). Two years later, while in Italy to start his plan to destroy mafia, they run into Tsuna.

Story starts the day high school student Tsunayoshi Sawada had to prepare for shool trip to Italy. But he woke up dreaming of an alternate life where he was Vongola tenth.

During the trip, he tried to live between his friend Take-kun, Kyoko-chan, Hana-kun, Haru-chan (from Midori junior high) and Kensuke-kun of kendo club and the mafioso he encounters during his stay there.

Along with smoking bomb, who he ran into, they had to stop Mukuro's gang before they destroy Vongola using the bovino's firearm artisanal. And Hibari-san's bloodthirsty nature (who participate to the trip as the comittee disciplinary, of course). And the Varia because they're bloodthirsty as professional assassin. 

Later, after returning to Japan, the second part of his other part's life came and he easily compared this ability to Byakuran's mare rings. He also learned that Enma, the guy he befriended in Nanimori middle school due to an earth quake making him and his friends transfer there, is the tenth heir of Simon Famiglia. If in the other life, Vongola and Simon fight, here, Enma's Tsuna's best friend.