Perhaps you misunderstood, Mormons do believe that God has a tangible body (See Luke 24:39), and that at some time he may not have been God. To fully understand that apparent contradiction requires an infinite mind, and we have something quite less than that in our present state. However, if you refer to John 5:18-19 you will see that Jesus is explaining to the Pharisees that he is only doing what he has seen his father do. The implication is God also had a mortal life where He did what Jesus did during his mortality. Since we know that Jesus grew from grace to grace, the assumption is God at some time also did.
As I mentioned an apparent contradiction about how God has always been God, yet could not have eternally been God if he went through a mortality like Jesus, requires us to re-examine our understanding of the meaning of eternal and eternity.
None of this belief on my part in any way (IMHO) diminishes the nature and stature of God. I only presently lack the ability to adequately communicate my spotty understanding of His nature.