For now, let ASSUME, both TYL!Reborn and TYL!Tsuna are DEAD... Please just PRETEND like THAT.

Because the TYL!Tsuna did destroy the Vongola Rings, so the Millefiore plan to use the time machine to travel to the past for getting the Vongola Rings. And the TYL!guardians are aware that.
Just image the TYL!Guardians are making a plan to counter like ...

"First, we need someone go to the past that can be able to bring/shoot the younger!guardians and their Vongola Rings to the future."

I bet TYL!Gokudera is the one who thought of that plan. Well, he's smart and he's the right-hand man after all. He likes to think of plan first right?
I even image that he could ask TYL!Hibari:

"Hey, Hibari, which is the easiest time that we could be able to shoot you without noticing?" "You're the f*cking hardest one after all."

Hibari ponders that question, "Uhm... Probably at noon on Friday after I finish my paper work in the reception room, I may be so tired and sleep well." "But I want you to shoot me one week later, I want bite to death some Millefiore herbivores before switching."

I think that's the reason that TYL!Hibari knows well the time he's gonna be switched.

Then the last question was set, "Who's the one will go to the past?"

*Sigh & Silent* There's no one answer...

If TYL!Tsuna and TYL!Reborn are faking death then it could be easier, because they are obviously the mastermind of this bring-my-younger-version-to-my-chao-future-plan.

Remember that TYL!Ryohei acted by TYL!Tsuna's order going to Varia's place and convey TYL!Tsuna's message. If TYL!Tsuna's the one who plan this, then message was something about "Dear Xanxus and Varia-tachi, our past selves are gonna come to the future and counterattack the Millefiore in Japan together by the time you guys plan to counterattack the Millefiore in Italy. Arrivederci :)" That must be my crack but it could be true.

If it's possible, you can go to my livejournal and read a whole mess of theories and questions that I made, but still had not been answered yet from the time I made that theory to the present time that chapter 205 was out :(

In my theory, Reborn is NOT dead (Tsuna maybe dead) and he is the one who's responsible to sent the younger!guardians to the future. Because he's the one whose TYL!self didn't appear after being shot by the 10-years-bazooka. Like I said, there's NO WAY that his future body didn't appear unless he's already in THERE, in the PAST (or his body turned to ash. But this seem silly to me). By the way, when Reborn of the past was going to the future, he end up in the base right? Or somewhere outside the base until TYL!Yamamoto and Giannini found him? Or maybe, they already expected and waited for Reborn of the past coming? Or maybe they already told him about the counterattack plan bring-the-younger-version-to-the-future. That's why he can be calm like that. Remember Reborn was sent to the future, one day after he met the younger Tsuna in the future in his special ridiculous cosplay. He said "Giannini made it for me, and the barrier at the door was also made it for me." I wonder if Giannini already made that before Reborn come to the future or after he come. This seems fishy.

The guardians cannot go to the past because they must stay in their planned place to be sure that their younger versions come to the future safely. That's why Tsuna's coffin wasn't buried. TYL!Gokudera with his boxes and G-Script instruction stayed near Tsuna's coffin. Lal wandered around nearby, waiting for the younger one appear. TYL!Yamamoto found Reborn and rescued the younger team. TYL!Hibari while investigating about the boxes, making plan with Mukuro, came back to rescue and train the younger version. TYL!Chrome aslo stay at her Kokuyo place.

I may remind you why TYL!Gokudera gave his sister, TYL!Bianchi, his Systema C.A.I. but not keep it? He only keep his skull cannon box with him. For what reason? Because it was PLANNED. First, if he was switched, in case of emergency, his younger!self can use the skull cannon box faster than think about how to use the Systema C.A.I. right? He gave that to his sister which means he wants her to train his younger self.

Everybody said something about the overshocked on Gokudera's face when he first met the younger!Tsuna. I can think one more theory, that the younger Tsuna arrived SOONER THAN that was PLANNED. Well that could be true, because Tsuna used the 10-year-bazooka to shoot himself, don't you remember? Of course, TYL!Gokudera didn't expect that. Of course, he's shocked, but he's calm down after that right? "I know you would come sooner or later, but not today, not too soon, I had not been ready to face you yet, Juudaime."

Yamamoto at his younger self is really the one who knows everything with the a look of innocence. TOO INNOCENCE. That's why I don't trust his TYL!Yamamoto who seems knowing something but just laughed it away and didn't tell anything X( I think in the future era, TYL!Yamamoto is the most pitiful and weakest guardian at all. He lost his father, not be able to smile happily and not be able to synchronize with his Shigure Kintoki. I think that's the reason Hibari always look after Yamamoto. ^__^ 8018 in here.

There're people talking about the Bovino's great evil plan to control the time. Now, let review about the past... in the Daily Life arc and Vongola Ring arc...

First about the the "Young Vongola" that TYL!Lambo called Tsuna while his child self call him "Tsuna" more closet and familiar. I think it's just the way he shows his respect. In 10 years later, Tsuna is his boss right? Because he from the Bovino family so he already has ONE Bovino BOSS in THERE, so obviously, he cannot call Tsuna "Boss" one more. A Mafiaso cannot have 2 bosses at the same time right? It could be called "traitor" And of course, Tsuna has to be in higher level than Bovino Boss. He's the Great Vongola X after all. ^__^ So no "Boss" for Tsuna (only Chrome can call him that) and "Juudaime" is only for Gokudera, his right-hand man to call Tsuna. Because he's younger than Tsuna, so he cannot call him "Tsuna" like he was in his child-self. So the option left to call him "Young Vongola" (because at that time, the 9th was still alive right, for "Old Vongola") ^__^ Remember when the 20YL!Lambo appears, he call Tsuna "Vongola X" (at this time there's no Vongola 9th) Problem solved.

Before that fight between the one who hold the Thunder Ring, Tsuna met TYL!Lambo and talk about the Ring Fight. TYL!Lambo said he had no memory collection about that ever and there's no rings in his era. Assume the time is the straight line, when we use the 10-years-bazooka to switch between our present self and future self, the only change is the point, not the line. So the progressing time between the present to the future should be fixed. Like TYL!Lambo said, if his child self died in the ring fight, his adult could not exist either. But thers's possiblity that his adult self could died in this battle. Just like you live until you 15 years old and then you go back to the past and die (Treat the past as some place on the earth) while your child self is still alive in the future. So basically, I think at that time, his child self is too young and in the confusion state that could not remember anything at all. And there's no ring in TYL!Lambo's era. Because, 9 years 10 months or maybe before that, TYL!Tsuna destroyed the Ring (Millefiore arc). Next, when the 20YL!Lambo appears, he said he remembers about the Ring Fight and was looking for his horns. This is because his TYL!self just finish his fight and run away using the 10-years-bazooka right before that and went to the era of 20 years later. So we can assume like that, after 5 minutes, TYL!Lambo who in the era of 20 years later went back to his 10 years later era and spend 10 more years training and went back to 20 years ago right on the day of the fight for Thunder Ring. So of course, he could remember. Just like going a trip. Actually, there's possibility that someone in the future tell him about the Ring Arc, may be Fuuta.

For the Bovino boss sending the 20YL!Lambo's horns to Iemitsu and 20YL!Lambo didn't know about where they could been (he was looking for them right?), it's really a mystery. I just wonder maybe Bovino boss is also the time traveler so that why he know the future. He went to the 20 years future and took 20YL!Lambo's horns because he does not want Lambo died in this era. Because, in this world, if we know the future, we could be the god of the world, right? Well, the Bovino is the first one who create the time machine that's the 10-years-bazooka. But it only lets them go to the future for only 5 min. So it's basically useless. So they need a greater mind that can able to invent the prefect time machine that can go both the future and the past without the time limit. And that's why they're messing with the young Irie for 2 reasons, terror him with the Vongola power and rise his interest in time machine. They possible know something about his intelligence in the future. Like Spanner said he met him at the Worldwide Robotic Fair Contest or something...

By the way, if the second post is true, I thought of one more theory, there's more than two 10-years-bazooka. First is the normal 10-years-bazooka that switch between the present and the 10-years-future in 5 min. Second is the modified one which switch between the present and the 9-years-10-months-future with no time limit. And it was the second that shoot the young!guardians around. And this one's not owned by the child!Lambo. But Lambo used THIS SECOND bazooka to shoot/sent Reborn to the future. So somebody else there switched it. Maybe the Bovino, oh my now this lead are they the mastermind in this mess? Or maybe the Vongola? Or may be BOTH?

That's all I thought after I read your theories and everyone's comments.

In chapter 205, Hibari's already wearing his Cloud Vongola Ring. This seems unusual. He usually wears it on his neck or may be tossed it away. Or maybe in the Sky Ring Fight, he did see his ring made a pretty purple flame so he tried out curiously and succeed and was amused. That could be the cause, I don't think that the TYL!Guardian would meet up with the younger!Guardians and tell them about their mess future at all. Or maybe, they could go to meet Dino and tell Dino to train Hibari so that he could make a flame on his ring. That's not a bad idea right? XD I know what the TYL!Guardian do in the past. Ha ha! I know, they gonna invest their fund for making their Vongola base. Beat to death Irie & Byakuran. Destroy the Namimori shopping center.