What still isn’t clear to me is why exactly this change will occur once more than half of world Jewry is in Israel. Why not once all are? Why not once we have the Beit haMikdash? I’m wondering how specific charedi streams will react to these changes in Halacha...

What we wrote is simply quoting the Chazon Ish (Shvi’it 21, 5), who explains that the universally accepted rule throughout the Torah is that one follows the majority (based on: Shmot 23, 2), see for example, in our context Rashi on Ktuvot 25b, that in the time of Ezra the Talmud says that all of the Jews did not come on Aliya, and Rashi explains “most” of them stayed in Bavel. Similarly, see Avoda Zara 36b, that a gzeira (rabbinical amendment) is only valid if all of Israel accepts it, and the gmara explains that, as usual, the majority is enough. The Chazon Ish is actually the most accepted halachic authority in the haredi world, and there’s no reason they shouldn’t accept his ruling. That doesn’t mean to say that all haredim know his psak on the issue, but the learned ones do, and I give them enough credit that most of them will accept the truth, even if it may contradict their “political” or philosophic view (I’m not naïve and know very well that some will look for excuses like: “there must be many Jews in the diaspora who don’t even know they are Jewish” and overlooking the fact that many who were counted Jewish there, aren’t halachically Jewish. Also, halacha doesn’t take into account Jews who are “lost”, for there almost always were and will be, e.g. the Lost 10 Tribes!). BTW, even the non-Zionist haredim differentiate between the holiness of the Land of Israel and her mitzvot, which concerns them very much, as opposed to the State of Israel, of which many oppose or support to different degrees. Also, even a non-Zionist who doesn’t recognize that we are in the process of national redemption, has no choice but to accept the halachic ramifications whether he likes it or not, just like the haredim stopped tearing their clothing when coming to the cities of Judea since 1948/5708 and 1967/5727 respectively, because we have a Jewish State and are no longer in a status of “churban” (destruction) but rather redemption (Mishna Brura 561, 2). You will never meet a haredi who tears his clothing since the Six Day War upon visiting Kever Rachel, Chevron, or Be’er Sheva, et al, but some may not know why there has been a halachic change! The Beit Hamikdash (Great Temple in Jerusalem), has certain ramifications regarding sacrifices (although they also can be revived without the Beit Hamikdash), but almost none regarding the 58 agricultural mitzvot. It's just that simplistically, many don't differentiate between the return of Israel, the rebuilding of the Beit Hamikdash and the Mashiach, which are mistakenly seen as one, when in reality they are clearly different stages (Megilla 17b) with totally different ramifications.