JEREMY - If God is unchanging why would he change something so sacred as the temple ordinance, and why have so many things changed in the church..polygamy, priesthood..temple stuff etc...if God is unchanging.

JOEL - There have been many changes over the years that have been required to accomodate the growth of the church and according to when God decides we are ready for more doctrine. He might also decide that there are some doctrines we don't need to live right now. For example plural marriage is an eternal doctrine, but but God has commanded that we don't live it right now. The same is true with other doctrines and principles.

"Wherefore I, the Lord, command and revoke, as it seemeth me good; and all this to be answered upon the heads of the rebellious, saith the Lord." (D&C 56:4)

God is unchanging, but we are not. We are constantly changing; hopefully for the better, and God will give us more when we are ready for it or He will take away something if we don't need it anymore or are unworthy to continue to have it.
The Book of Mormon summarizes this principle as follows:

"For behold, thus saith the Lord God: I will give unto the children of men line upon line, precept upon precept, here a little and there a little; and blessed are those who hearken unto my precepts, and lend an ear unto my counsel, for they shall learn wisdom; for unto him that receiveth I will give more; and from them that shall say, We have enough, from them shall be taken away even that which they have." (2 Nephi 28:30)

The ordinances and basic gospel of Jesus Christ do not change. But the way they are taught may need to change depending on the time and circumstances. Let's take the Temple ordinance for example. I can't discuss any specific details outside the Temple, but the temple ordinance has been changing ever since it was first introduced, according to the needs of the saints at a particular time in history. The ordinance used to take the better part of an entire day to get through; now it takes much less time, yet the promises and covenants we make have not changed at all. It is important to remember that the temple ceremonies are teaching mechanisms, designed to help us make promises to God. Therefore, they will be changed in accordance with the spiritual growth or regression of the people. As a people grow in spirituality, lesser teaching may be taken out and greater ones added. If a people regress spiritually, teachings may be lost or made less plain (Jacob 4:14; D&C 43:10; Matt. 13:10).

Here is what Joseph Smith said about the eternal nature of the temple ordinance:

"Ordinances instituted in the heavens before the foundation of the world, in the priesthood, for the salvation of men, are not to be altered or changed. All must be saved on the same principles." (TPJS, 308)

The "principles" of the temple ceremony that make it an "ordinance" have not changed. We are all "saved on the same principles" and covenants we make with God found in that ordinance regardless of how they are taught to us in the temple ceremony.
There is an interesting article about this at this site.