Different religions have different ways of looking at the 144,000, most viewing them as representatives of the twelve tribes of Israel, 12,000 from each tribe. These will serve as representatives that go out into the world to gather the remnants of Israel prior to the return of Christ. These will not be appointed forever, but only until their missions are complete. The Jehovah's Witnesses employ what's called replacement theology in which as all New Testament references, promises and blessings to the twelve tribes of Israel are replaced and given instead to the body of Christ, that is, the church. It is used by a number of Christian sects, mostly those that were formed before the signing of the Balfour Declaration (November 2, 1917), which was the beginning of recognizing Palestine as the reestablished nation of Israel. Isaiah and other prophets had prophesied Israel would be gathered from the nations of the earth (see Isaiah 11, for example).
But the Jehovah's Witnesses were formed before Israel formally became a nation in 1949. And since Israel did not exist, replacement theology was the only logical fulfillment of these prophecies. Sadly, when Israel became a nation, their doctrine was already set in stone! Thus, if they were to reapply those prophecies to Israel, it would make them appear to be revamping their eschatology.
John wrote: “And I looked, and, lo, a Lamb stood on Mount Zion, and with him an hundred forty and four thousand, having his Father’s name written in their foreheads.” (Revelation 14:1) According to both the Old and New Testaments, when Yahweh returns to the earth, he will stand upon the Mount of Olives (Mount Zion). See Zechariah 12, 14 and Revelation 11. The two prophets raised up to the Jewish people at that time have just been killed, but after three and a half days life enters into them and they stand upon their feet and ascend to meet the returning Savior. Jesus' feet stand upon the Mount of Olives and with him the 144,000.
The JWs have centered their entire doctrine on themselves, however, and this is a problem. It's best to read the scriptures and try to piece them together with a clean sheet of paper. If you begin a journey with a bad map, where you begin and where you end up will be in error. As I read some of their prophecies, and their exegeses, I'm flabbergasted at the liberties they take.