John the Baptist is most likely the answer you are looking for.
Jesus
Jesus' death opened a new covenant with God.
No Jesus did not convert. The Old Testament tells us of a new covenant that God was giving to the Jewish people in "time to come". When Jesus (the Son of God) came to earth in human form that "time had come" Jesus simply instituted the new covenant. It was the Jewish people who failed to recognize the Messiah and follow Him into the new covenant.
Yes, and the covenant is in his blood, as the Scripture says in Hebrews 12 -".22 But ye are come -------- 24. to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than [that of] Abel.
The promise of the New Covenant has been given to the remnant of the Jewish nation. For all intents and purposes the New Covenant has been received by the believers in Jesus Christ.
After the death of Jesus. Paul taught of the life and works of Jesus Christ and his teaching of a New Covenant
Allow me to refer you to the New Testament epistle to the Hebrews. It addresses this very question, especially chapters 7-9.
The New Covenant took effect the same minute that Jesus Christ died. The New Covenant erased the Death Penalty either by stoning or any other method invented by man. The New Covenant is based on Mercy and Forgiveness and 2nd Chances. Unfortunately 95% of the Jews rejected Jesus as the Messiah and son of God by asking the Roman Empire to kill Jesus. The Jews have until this day of the 21st Century innocent blood in their hands for asking the Romans to kill Jesus so the Jews are not God's people at all.
God made many covenants in the Old Testament, but I assume you are speaking about the "New Covenant" vs the "Old Covenant" which God gave Israel through Moses at Mount Sinai. The "New Covenant" or second was mentioned in the Old Testament several times as a covenant which was yet future. The "New Covenant" started in the New Testament when Jesus Christ died on the cross of Calvary, was buried, and was resurrected the third day. The 'Old Covenant" was by Law and the "New Covenant" was by grace.
The New Covenant is a covenant that God has made with the remnant of Israel and has not been fulfilled yet, (Jeremiah 31:31-34) although it is true that the Christians enjoy many aspects included in the New Covenant the covenant was promised to the Jews. I don't know if a symbol is involved or not. The Holy Spirit of God will be given, but He is more than a symbol.
Jesus Christ, the Messiah of Israel.
As for the old covenant we fell under the Mosiac Law and now for the new covenant we fall under the grace of our Lord and Savior Jesus Christ. Answer To get a fuller understanding of this subject, read the book of Hebrews in the Bible,it might take you an hour.... tops. It was wrote primarily to explain that very thing, and how the new covenant that replaces the old is a better permanant one that replaces a temporary one that was a foreshadow of the new one that was to come. Notice in your reading of Hebrews the key words of : better, and, covenant. It will all be spelled out to you.