From what I understand of premillenial eschatology, during the millennial reign, people who survived the tribulation will have their longevity returned like before the flood, but they will still die. What happens to those that die, but are saved? Jesus is already on earth with the saints, the people saved before the tribulation; but what about those saved after? Would they just get their glorified bodies immediately?
Here's how I understand it. The people who have become believers during the Millennium Kingdom won't die! Think about it. We know from Scripture that the people who make up the population during this time are those who were martyred during the Tribulation and came back to life to reign with Christ, and those who didn't take the mark of the beast, instead choosing to believe and accept Christ and survived the Tribulation. The other inhabitants are the Saints-the dead in Christ who were resurrected and those raptured before the Tribulation. They will also rule and reign with Christ. The people who are still in fleshly bodies will be like as in Noah's day, eating and drinking, working and playing, marrying and having children. Everyone will live long, long lives. The age of accountability will be extended to a hundred years! There will be no babies or children that die. All who are born during that time will have to choose Jesus (be saved) just as before the Millennium. Those who don't accept Christ will be cursed. If they die during the Millennium, they will go to Hades to await the final judgment. If they remain alive until the thousand years over ended, then they will have an opportunity to accept Christ or participate in the last rebellion when Satan will be loosed for a time to deceive the nations. There will be absolutely no fence sitting at this time. After the thousand years are ended, the rest of the dead will come back to life and be judged. Read Rev. 21 for what happens next!
The passage you link to is Isaiah 65:20:
Your assumption is that this is referring to Christians. However, we know that during the Millenium there will be Christians and non-Christians (either those that come out of the Tribulation or perhaps people born during these 1000 years):
So my understanding is that Christians will all have their resurrected, immortal bodies during the Millenium, and non-Christians will not.
Unfortunately, this theory does not address what happens to people who become Christians during this time!