Zion was intended to be built in the center of the last part of Lamanite land available in 1831. The fact that the gentiles were expelled does not mean the site for building Zion was automatically changed. The Lord reiterated Zion wasn't changed. The gentile children may be scattered, but the site would remain. More importantly, the Lamanite children were being scattered as well. The picture was changing on both sides of the line separating "Jew from gentile" in the years following the 1831 revelation.
By 1838 the conflict between Mormons and Missourians had escalated to the point that it was called the "Mormon War." The election battle at Gallatin on August 6, 1838 is at one end, and Joseph Smith's surrender at Far West in November, 1838 at the other.
Missouri was lost to the Saints. The natives voted to expel them, and Governor Lilburn Boggs signed the Extermination Order on October 27, 1838 requiring Mormons to be exterminated or driven from the State of Missouri; a curious piece of Americana that was not rescinded until some 137 years later on June 25, 1976 by Missouri Governor Christopher Bond.
The immediate aftermath of the Extermination Order was the battle at Haun's Mill, ultimately leading to the surrender in November by Joseph Smith. He was subsequently tried by a military tribunal and sentenced to death, but the death sentence was not carried out.
Joseph spent the winter of 1838-39 in the Liberty Jail, and in March, 1839 wrote a letter from which we have taken three sections of the Doctrine and Covenants, Sections 121, 122 and 123. The possibility of building in Missouri was lost, at least for the time.
The Saints moved to Commerce, renamed it Nauvoo, and started a new city. This one was also identified not only as "Zion" but as the "cornerstone of Zion." (D&C 124: 2.) So, although "Zion" was not to be moved, by 1841 the "cornerstone of Zion" was now in Nauvoo. This is not a contradiction. Zion has never been moved. But the Lamanites were moving, the Saints were moving, and the opportunity to locate it in the places where it could have been constructed earlier were no longer relevant.
We read the words of Section 101 to mean that the location remains in Independence, Jackson County. It is possible, however, there is another meaning. That is, the location hasn't changed, although temporary opportunities existed earlier. It wasn't built earlier, and will be built, but when it is built, it will be at the place always prophesied for its construction. Zion was to be located on the top of the high mountains. (Isa. 40: 9.) Jackson County has no mountains, no mountain range, no possibility of fulfilling the promised environs for establishing Zion. (Isa. 2: 3.) Make the descriptions "spiritual" if you want, but a mountain setting is clearly required for the prophesied Zion. (Psalms 133: 3; Isa. 52: 7; Joel 3: 17; Micah 4: 2; 2 Ne. 12: 3; D&C 49: 25; among others.)
Zion was always intended to be built upon the mountain top. (Isa. 30: 17.) Even a valley location in Salt Lake cannot answer to the description given in prophecy. A valley floor is not the "top of the mountain" upon which the beacon will be set. Zion has never been moved. Nor will it. In the same revelation which confirms Zion will not be moved, the Lord spoke of the Saints profaning the land earlier identified as Zion. "For all those who will not endure chastening, but deny me, cannot be sanctified. Behold, I say unto you, there were jarrings, and contentions, and envyings, and strifes, and lustful and covetous desires among them; therefore by these things they polluted their inheritances. They were slow to hearken unto the voice of the Lord their God; therefore, the Lord their God is slow to hearken unto their prayers, to answer them in the day of their trouble." (D&C 101: 5-7.)
So the location identified for building Zion was lost. It was lost because of the jarrings, contentions, envyings, strifes, lustful and covetous desires. This caused the land to be "polluted" and rendered it unfit for Zion. It is true, however, that in the same revelation making purchase of land in Jackson County was approved. (D&C 101: 70-71.) There is no doubt a glorious future for Jackson County. But that will be by and by. There is a gathering in the tops of the mountains which must precede that. If there is not a gathering in the mountains first, then ancient and modern prophecy will fail. There is to be a gathering within the boundaries of the everlasting hills. (D&C 133: 31-32.) Zion will flourish upon the mountains. (D&C 49:25.) There aren't any places in Missouri that qualify for this preliminary gathering.
If jarring and contending can pollute Zion, are we ready for it now? If envy and strife will make it unacceptable, how prepared are we to gather to Zion now? If lustful and covetous desires will make it unfit for an inheritance, are we above those weaknesses now? So, how soon ought we expect the establishment of Zion to get underway?
All of this is an aside to the subject of the remnant. But it is an important aside. The remnant will build the city of Zion. In 1830, when the earlier inhabitants were relocated to the area immediately adjacent to Jackson County, had the city been built it would have been there. It wasn't time. It also wasn't the place. So, although the future of that place may be glorious at some point, the city of Zion to be built by the remnant, would necessarily be built where the remnant is located. Their location, if it answers to the description of prophecy, would be mountainous, in the top of the mountains, and a suitable place for refuge during a time of upheaval. We'll follow the events of the 1840's with that in mind.