Immutability & the Incarnation

I have been listening to mp3s of a Systematic Theology I class from Reformed Theological Seminary, taught by Dr. Douglas Kelly. Though I have never attended such a class in person, nor have I taken one for credit, I have a great interest in systematics of all sorts, and in theology in particular. In many systematic theologies, one begins with the topic of The Bible, its nature, and transmission. You would then discuss Theology Proper, God as one Essence and three Persons, His attributes and relation to His creation. It is in this section that I want to linger for the purposes of this article. One of the characteristics attributed to God is that of immutability, or un-change-ability. The word itself comes from the Latin, in, meaning "not", and mutare, meaning "to change". Regarding the definition and intentions of systematic theology, B.B. Warfield is helpful:

"The task of systematic theology is not to validate the reality, or to define the nature, or to determine the method of revelation; nor, indeed, even to ascertain the truths communicated by revelation; but to systematize these truths when placed in its hands by the precedent disciplines of apologetical, exegetical, and historical theology. Systematic theology is thus, in essence, an attempt to reflect in the mirror of the human consciousness the God who reveals Himself in His works and word, and as He has revealed Himself."

The other topic I want to intersect with the doctrine of the immutability of God, is His incarnation.As far as systematic go, this topic might be introduced under the category of Trinity, and others develop a separate tmajor topic titled, "Christology", and they may include under that category, the doctrine of incarnation. In any case, the question that begs to be asked when considering the Bible's teaching of God's unchangeable character and the Christ becoming a man is this, if God never changes, yet before the incarnation He did not have a permanent body as we do, then how does He maintain a character of immutability after Christ came to Earth, died as a human man, was raised bodily, ascended to Glory, and now exists with a glorified body?

One mention of this supposed paradox or tension is in George Lawson's book of 1813, "Discourses on the Condition and Duty of Unconverted Sinners...". Dr. Dennis Johnson of Westminster Theological Seminary in California covers this topic in his writing, "Immutability ans Incarnation". Unfortunately, I have not read this work, but I do intend to. Calvin touched on the subject briefly during his controversial dealing with Servetus. I'd just like to open this topic up for discussion.