Whose Teva?

In the time of Techiyas HaMaisim, all of the tzadikim will return to the world. Question is whose world will they return to? For instance, the Rambam writes 2 perakim in MT, hilchos Dayos on good health practices, which are different than those written in the Talmud. Many of these health practices and facts are different than what we find today. The obvious answer is that nature has changed, nishtaneh hateva. These matters are halacha, so that good health is an obligation, not just avoiding illness. One could argue that cures are not relevant after Techiyas Hamaisim, but optimal health would still be.

The question is, when Talmudic Rabbis return, will they return to current nature or to nature as it was in their times? Will they require different halachas? Based on what? And if we change the halacha based on their nature, why is it anathema that we change halacha based on our changed nature? Why is that a nonfrum concept?

And while we are at it, will the planets continue to go around the sun or will they revert to nature at the time of the Rambam and go around the Earth, well roughly anyway? Or will nature revert to the Talmudic time and have the sun go through the rakia, eliminating all time zones and unifying the calculation of the entry and exit of Shabas for the entire world?