Chaim, the Torah is eternal and immutable. It won't change in the world to come. All Gentiles are forever obligated to die by the Hand of Heaven if they observe the Sabbath.
I think Rambam's wording is like "for all peoples" in Isaiah 56:7. Rashi explains: "Not only for Israel, but also for the proselytes."
ืืก''ื
The Torah certainly is eternal and immutable. But how the commandments are observed and interpreted can change if the Sanhedrin (the court of 71 sages seated at the Holy Temple) decides to do so. We are obligated to obey the rulings of the Sanhedrin.
For example, there is a Midrash (Oral Torah teaching) that tells us that all of the current Jewish holidays (including those specified in the Torah) will be nullified in the future except for Purim and Yom Kippur. Heres's the original Hebrew Midrash followed by the translation:
ืืืจืฉ ืืฉืื ืืณ:ืืณ
...ืื ืืืืขืืื ืขืชืืืื ืืืืื, ืืืื ืืคืืจืื ืืื ื ืืืืื ืืขืืื, ืฉื ืืืจ (ืืกืชืจ ื ืื): "ืืืื ืืคืืจืื ืืืื ืื ืืขืืจื ืืชืื ืืืืืืื". ืืืจ ืจืื ืืืขืืจ: ืืฃ ืืื ืืืืคืืจืื ืืื ื ืืื ืืขืืื, ืฉื ืืืจ (ืืืงืจื ืื ืื): "ืืืืชื ืืืช ืืื ืืืืงืช ืขืืื ืืืคืจ ืขื ืื ื ืืฉืจืื ืืื ืืืืชื ืืืช ืืฉื ื"...
Midrash Mishlei 9:1
...All of the holidays are to be nullified in the future but the days of Purim will not be nullified, as it is stated (Esther 9:28), 'And these days of Purim will not be rescinded from the Jews.'" Rabbi Elazar said, "Also Yom Kippur will forever not be nullified, as it is stated, 'And it will be to you for an everlasting statute to atone for the Children of Israel from all of their sins once a year.'"
Does that mean that part of the Torah will be nullified? G-d forbid! But how we observe the holidays will change when the Sanhedrin decides to do so.
I could go into much more detail about the Gentiles in the next world but will be brief. First of all, there is a difference between bnei Noach and other Gentiles. Bnei Noach are on a much higher spiritual level because they understand that the commandments were given to them by G-d. There is a concept in Judaism of continuous spiritual elevation in the next world. The righteous Gentiles who are granted eternal life will not stand still in the next world. Their spiritual elevation will include eventually adopting other commandments.