Isaiah 53 was prophecy of what was to come, the Church, with the coming of the Messiah who is Jesus.
1 Peter 2:9 But you are a chosen people, a royal priesthood, a holy nation, God’s special possession, that you may declare the praises of him who called you out of darkness into his wonderful light.
I refer to your original post.
A question I’ve always wondered about?
The Messiah of the OT is nothing more than a man who will be the king of Israel when utopia comes to Earth.
How did that Messiah change into the Messiah of the NT?
I suppose I don’t quite understand what you are looking for in an answer.
The Messiah didn’t change, rather the Messiah was not recognized by the Hebrews or the nation of Isreal. They were guessing who was the Messiah even as the Messiah stood before them as Jesus the Messiah. I might add Jesus was always there but not known until He came in flesh. Just as Jesus is with us now but we know who Jesus is.
Peter’s Declaration about Jesus
Mark 8:27 Jesus went on with his disciples to the villages of Caesarea Philippi; and on the way he asked his disciples, “Who do people say that I am?” 28 And they answered him, “John the Baptist; and others, Elijah; and still others, one of the prophets.” 29 He asked them, “But who do you say that I am?” Peter answered him, “You are the Messiah.” 30 And he sternly ordered them not to tell anyone about him.
As I understand the Jewish people still await their Messiah who we believe is Jesus who fulfilled the prophecies. The Jews awaited an earthly King to lead them out to the Promised land yet the Kingdom of God is not of this world. The promised land is heavenly Jerusalem when there is a new earth and a new heaven.
John 18:36
Look at the definition of Messiah.
mes·si·ah
məˈsīə/Submit
noun
- the promised deliverer of the Jewish nation prophesied in the Hebrew Bible.
Jesus regarded by Christians as the Messiah of the Hebrew prophecies and the savior of humankind.
- a leader or savior of a particular group or cause.
“he was regarded as a messiah by liberal and conservatives alike”
Jesus even pointed the pharisees to the scriptures explaining to them that He was the Messiah.
Mark 12: The Question about David’s Son
35 While Jesus was teaching in the temple, he said, “How can the scribes say that the Messiah is the son of David? 36 David himself, by the Holy Spirit, declared,
‘The Lord said to my Lord,
“Sit at my right hand,
until I put your enemies under your feet.”’
37 David himself calls him Lord; so how can he be his son?” And the large crowd was listening to him with delight.
And also here where Jesus said:
John 5:39 “You search the scriptures because you think that in them you have eternal life; and it is they that testify on my behalf. 40 Yet you refuse to come to me to have life.
The deliverer is Jesus and always has been Jesus, The Word of the Lord.