"Appearance" involves the outward visible aspect of something or someone. In John 1:18 we are told that: "No one has seen God at any time".
Here God the Son, "He, has declared Him" [Him the Father]. So Him refers to both God who has not been seen and the One in whose bosom the Son is. If this is so then it is the Father who has not been seen. But the Son is seen as in 1 John 1:1 it says "we have seen with our eyes, which we have looked upon". Here the physical reality of Jesus is strongly affirmed. ..The incarnation is Jesus being seen in time but "Jesus Christ is the same ..forever" Heb13:9. Jesus does not change. ..Whatever attributes Jesus ever had He always had. He was seen by John He could be seen by Abram.
For the Lord to appear to Abram it seems to me that this was Jesus appearing. Is "the LORD" definitely Jesus in Gen.17:1?