You are arguing with one that thinks the Son of God had a physical body before creation. Such a thing is possible even if not plausible in the context of Melchizedek.
Hebrews 7:1 This Melchizedek was king of Salem and priest of God Most High. He met Abraham returning from the defeat of the kings and blessed him, 2 and Abraham gave him a tenth of everything. First, the name Melchizedek means “king of righteousness”; then also, “king of Salem” means “king of peace.” 3 Without father or mother, without genealogy, without beginning of days or end of life, resembling the Son of God, he remains a priest forever.
But, Jesus was conceived by the Holy Spirit in Mary and born a child that grew in stature, physical and mental. So if you try to use Hebrews 13:8 in a literal way applying to Jesus' physical body, it has some obvious holes. The reading without holes is to apply the verse in the context of Jesus' spirit which is clearly unchanging and eternal.
And in support, you have this verse where Jesus says he was around when Abraham was alive. Obviously he could not have been referring to his physical body.
John 8:58 “Very truly I tell you,” Jesus answered, “before Abraham was born, I am!”