In the New Testament the Greek word translated "new" in "new covenant" is, with one exception, kainos. It means "'new' as to form or quality, of a different nature from what is contrasted as old" (Vine's Complete Expository Dictionary of Old and New Testament Words, 1985, "New," emphasis added).
Only in Hebrews 12:24 Hebrews 12:24And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaks better things that that of Abel.
American King James Version×is the different Greek word neos used for "new" in "new covenant."Neos "signifies 'new' in respect to time, that which is recent; it is used of the young, and so translated, especially the comparative degree 'younger'; accordingly what is neos may be a reproduction of the old in quality or character" (ibid., emphasis added).
Neither Greek word translated "new" suggests that every aspect of the Sinai or Old Covenant was replaced. Each only indicates that the more recent covenant has enhanced and improved the quality of the original covenant.
The New Covenant unquestionably provides a better relationship with God than the relationship portrayed only symbolically in the former covenant. To ensure this better relationship, some new features have been added to the "new" (or qualitatively improved) covenant and some obsolete features have been replaced. But features common to both covenants remain unchanged and unaltered.
That new relationship is available only through Jesus Christ, our new High Priest and our real sacrifice for sin. Christ's death, by paying for humanity's sins, has opened the door for all who willingly repent to receive God's Holy Spirit and be accepted by God as His sons and daughters. As our permanent High Priest, Jesus replaces the high priest who was merely a human descendant of Moses' brother, Aaron.
God's "new" covenant also offers enormously better promises. Yet it abandons none of those spiritual principles that eternally reflect God's mind and character. Those principles are explained accurately and adequately enough in the Old Testament scriptures. Jesus and all of His apostles, including Paul, used these Scriptures as their authority for the true teachings of God (Matthew 4:4 Matthew 4:4But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceeds out of the mouth of God.
American King James Version×; Acts 17:2 Acts 17:2And Paul, as his manner was, went in to them, and three sabbath days reasoned with them out of the scriptures,
American King James Version×; Romans 1:1-2 Romans 1:1-2  Paul, a servant of Jesus Christ, called to be an apostle, separated to the gospel of God,  (Which he had promised before by his prophets in the holy scriptures,)
American King James Version×; 2 Timothy 3:14-17 2 Timothy 3:14-17  But continue you in the things which you have learned and have been assured of, knowing of whom you have learned them;  And that from a child you have known the holy scriptures, which are able to make you wise to salvation through faith which is in Christ Jesus.  All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness:  That the man of God may be perfect, thoroughly furnished to all good works.
American King James Version×).