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?
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!