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Question: Were the changes to the revelations hidden from the Church members?
Oliver clearly understood that some changes were corrections, and some were additions given by revelation which were made prior to publication
One aspect of the prophet's mission includes the editing and modification of revelation prior to publication. There is plenty of evidence that the Church has done nothing to hide the fact that changes were made.
This information has been available since the first publication of the revelations which later became the Doctrine and Covenants. The Saints of Joseph Smith's day had read the revelations in their initial form, many having been published in Church newspapers. Oliver Cowdery wrote, upon the publication of the revised revelations:
On the revelations we merely say, that we were not a little surprised to find the previous print so different from the original. We had given them a careful comparison, assisted by individuals whose known integrity and ability is uncensurable. Thus saying we cast no reflections upon those who were entrusted with the responsibility of publishing them in Missouri, as our own labors were included in that important service to the church, and it was our unceasing endeavor to have them correspond with the copy furnished us. We believe they are now correct. If not in every word, at least in principle. For the special good of the church we have also added a few items from other revelations. 
Oliver clearly understood that some changes were corrections, and some were additions given by revelation which were made prior to publication.
Orson Pratt noted that "line was added upon line to several of the sections and paragraphs about to be published"
In 1854, Orson Pratt discussed changes:
We often had access to the manuscripts when boarding with the Prophet; and it was our delight to read them over and over again, before they were printed. And so highly were they esteemed by us, that we committed some to memory; and a few we copied for the purpose of reference in our absence on missions; and also to read them to the saints for their edification. These copies are still in our possession. When at length the time arrived to print the manuscripts, it was thought best not to publish them all, on account of our enemies, who were seeking every means to destroy the Prophet and the Church…. It was concluded, through the suggestions of the Spirit, that by altering the real names given in the manuscripts, and substituting fictitious ones in their stead, they might thus safely appear in print without endangering the welfare of the individuals whose real names were contained therein….
It may be asked, had the Prophet a right to alter names given by revelation and substitute fictitious ones in their stead? We reply, that it is only the printed edition that contains the substituted names, while the original manuscripts, that are safely preserved in the hands of the church, contain the names as they were originally given. Moreover, the substitution of fictitious names for persons and places does not alter or destroy the sense or ideas contained in the revelations. But what the Prophet did in relation to this thing, was not of himself; he was dictated by the Holy Ghost to make these substitutions…. And by revelation line was added upon line to several of the sections and paragraphs about to be published.
But some may inquire, are not the Almighty’s revelations perfect when they are first given? And if so, where was the propriety of the Lord’s adding any thing to them, when they were already perfect? We reply that every word of God is perfect; but He does not reveal all things at once, but adds ‘line upon line, precept upon precept, here a little, and there a little,’ revealing as the people are able to bear, or as circumstances require…. The Lord, therefore, adds to His own revelations whenever he thinks proper.
"The Prophet was inspired in several instances to write additional sentences and paragraphs to the earlier revelations"
In 1857, the Millennial Star noted:
Joseph, the Prophet, in selecting the revelations from the Manuscripts, and arranging them for publication, did not arrange them according to the order of the date in which they were given, neither did he think it necessary to publish them all in the Book of Doctrine and Covenants, but left them to be published more fully in his History. Hence, paragraphs taken from revelations of a later date, are, in a few instances, incorporated with those of an earlier date. Indeed, at the time of compilation, the Prophet was inspired in several instances to write additional sentences and paragraphs to the earlier revelations. In this manner the Lord did truly give ‘line upon line, here a little and there a little,’ the same as He did to a revelation that Jeremiah received, which, after being burned by the wicked king of Israel, the Lord revealed over again with great numbers of additional words (See Jeremiah 36:32) 
- History of the Church of Jesus Christ of Latter day saints, 1805-1890, Volume 1, p. 580-81.
- Orson Pratt, “Explanation of Substituted Names in the Covenants,” The Seer 2.3 (March 1854): 227-9.
- ?, "Restoration of the Aaronic and Melchisedek Priesthoods," Millennial Star 19 no. 17 (25 April 1857), 260. (Scripture references modernized for wiki linkage.)