No the brethren haven't changed the revelations about doctrine given by previous brethren. The issue is faith. Practices may change as the Lord sees fit to adapt them to the righteousness of his people, (as they did in the scriptures as well) but truths do not, and have not.
The issue is faith.
Elder Scott taught that as we learn to rely on the Holy Ghost we will someday believe the whisperings of the Holy Ghost beyond what our eyes and ears tell us is true.
How can we know whether we are hearing correctly? By obeying the commandments and keeping our covenants. The more perfectly we are willing to align our will with the will of our Father, the more plainly he is willing to manifest his will to us, and the more we are able to exercise faith knowing that we are doing what the Lord commanded.
Thus the Lord's light grows brighter and brighter until the perfect day.
The dichotomy of choosing faith and obedience vs choosing to put aside faith, and thereby also choosing not to obey is all through the scriptures and is a defining one in mortality.
Lehi has a dream and tells his family to leave most of their stuff and go into the wilderness. Nephi has faith and listens. Laman and Lemuel believe they recognize how crazy this whole thing is and without faith, murmer and rebel.
They are sent back to get the plates. It isn't going well. Laman and Lemuel do not see it through eyes of faith, and are angry at their irrational father and brother. Nephi sees it through eyes of faith and knows that they can do whatever the Lord commands, and succeeds.
They are supposed to build a boat. Laman and Lemuel feel that for men who are not boatmen to try and cross the ocean is obviously suicide, but it doesn't matter because they know Nephi can't do it anyway. They see it without eyes of faith. And because of that they rebel, mock Nephi and won't help.
Nephi knows the Lord commanded him to build a boat. etc....
I mean, I could go on, and on, and on because the effects of faith, or of rejecting faith, is in every meaningful story in the scriptures.
Faith does not demand a perfect knowledge in order to act. The principle of faith is that when the Lord speaks to us, we listen and obey. Anything else is not faith.