But He had to be God from the moment of His conception or He could not have been Redeemer. That’s what He was called “God with us”: He was fully God even in that single cell in the God-Bearer’s womb.
The Old Testament knew nothing of “avatars”; it knew of the Kinsman-Redeemer, and the Son of Man who was seated at the right hand of Power, and the Messiah who was David’s son while David’s Lord, but not a thing about an avatar.
Yes, He deliberately as an act of grace did not use the knowledge that was His, since the Event of Redemption required He live as human, and so by setting aside divine prerogatives He lived as one of us, but He began as God and remained as God; after all, “by Him all things were made, and apart from Him was not anything made that was made”, since He was the Word Who became flesh and lived among us, Who was God already in the beginning.
The Redemption wasn’t just about the Cross and Resurrection, it was about God Himself living as fully human for an entire human live, thus taking on Himself everything about human life, not as something tacked on at the end but from the very moment when Gabriel announced to Mary that she would be giving birth to the Son of God, only-begotten of the Father from all eternity.
The change at the Resurrection was that His locally-limited human body became an exalted, glorified human body no longer limited by time and space, not that He was in any way promoted. He was already the “true, physical form of God” – He said as much to His disciples – but that physical form was itself transformed.
Perhaps oddly, while I’ve read many a page about the “pre-incarnate” Christ in the Old Testament, I don’t recall any writer bothering to distinguish whether that was His resurrected body or not. I suppose there isn’t much of a different choice except perhaps His transfigured body as when He met with Moses and Elijah; who knows what transpired on that mountain with them? My issue with it being the resurrected body would be that the holy wounds didn’t really belong on Earth until they were inflicted on Earth, and given that He was talking with two figures from the Old Testament who departed this Earth not according to the normal pattern I see no real issue with Him stepping off into time three (or seven, depending whether you count Him as the “Angel of the Lord” Who spoke as the Lord or just the times He was plainly described as a man) instances to appear to Abraham, etc.
Now I’m going to fall asleep puzzling over that one!