Simply put, Practices can change, but Doctrine is eternal. 
 
Practices can change as the Lord reveals things through His servants, the prophets. This shows how important it is to have ongoing revelation; someone who speaks on behalf of God. Many points of the Law of Moses were practices that were fulfilled with the death of Christ. 
 
However, every practice originates from a doctrinal parallel. The doctrine of the Gospel of Jesus Christ is unchanging, even though true followers of Christ in each dispensation practiced according to the revelation they received. 
 
For example, in Old Testament times, sacrifices were offered in the tabernacle to remind the Children of Israel of the coming of the Messiah, and of the Atonement. In original Church, the sacrament was instituted as a reminder of that great atoning sacrifice. The doctrine of the Atonement remained constant, but as the time period changed, the practices changed. 
 
I'm glad that God continues to reveal the order of certain practices to living prophets, and I'm glad we have their words and the scriptures to establish the doctrines of truth.