"Once the Arians implicitly introduce temporality into the Father-Son relation, they implicitly introduce temporality into the existence of the Father himself. If there is an interval between Father and Son, and yet the Son is somehow "God," then God is subject to becoming (Discourses 1.17). If the Triad emerges, then true religion is not fixed; piety is trying to hit a moving target (Discourses 1.18. For the Arians, God becomes Father only after begetting the Son (Discourses 1.24). The Arians imply in various ways that God is subject to time, that he is a God-in-process.
The point of the paradigm of sonship thus cannot be to highlight a temporal interval between Father and Son. To be sure, there is such an interval in human life, but since God is beyond time, there can be no interval." - Leithart, Athanasius, p. 51
If the Father is not eternally the Father (and He is not if there is not eternally a Son), then God the Father as we know Him now is not the same. The unchangeable has changed.