I have heard that after the tribulation, people who have become Christians will enter eternity as mortal people and will continue to live mortal lives, being sustained by the tree of life. New life will be perpetual in this sense. If this is not true, how are they changed or transformed after the tribulation?
At the rapture the Church will disappear from Earth. Partly because of this many of those left behind will become believers during Daniel’s 70th Week. The ones who survive the Great Tribulation will come into the 1000 year Millennial Kingdom (not eternity) as mortals. There’s no indication they will be changed or transformed in any way. The Bible doesn’t say how long they’ll live or what will happen when they die, but there is no reason to conclude that either they or their children will automatically become immortal. The references to the tree of life in Ezekiel 47:12 and Rev.22:2 mention sustaining people in Israel but don’t say anything about facilitating perpetual life on Earth. Everything after the Millennium is a mystery God has not revealed.