No where in the Bible does it say a sheep will always stay a sheep, in fact it says many will be led astray, some will fall away, some will be grafted out, some will be planted in shallow soil, and some will turn aside to follow satan, and some will run out of oil for their lamps.
I don't think it says anything about being "grafted out." It says we may be "grafted in" but not to boast against the branches (seed of David.) It also doesn't say that any of us per say will be "planted in shallow soil," but that our hearts may be shallow soil where the Word of God cannot fall in deep enough to take root. The solution to this is to plow the soil so that it will take the seed properly. Which is where many fall away.
(see Mark 4:1-20; Rom. 11:17-21)