Forsake not an old friend; for the new is not comparable to him: a new friend is as new wine; when it is old, you shall drink it with pleasure.
All Commentaries on Wisdom of Sirach 9:10 Go To Wisdom of Sirach 9
Fulgentius of Ruspe
The New Testament must be held in veneration in such a way that the Old Testament not be neglected in any way. This is what Ecclesiasticus seems to me to recommend under the guise of old and new friends when it says, "Forsake not an old friend, for a new one does not compare with him." From the mouth of Jeremiah, the divine Scripture anticipates the difference that is found in the mysteries of the two Testaments, saying, "The days are coming, says the Lord, when I will make a new testament with the house of Israel and the house of Judah, not according to the testament that I made with their ancestors." For the other, that is, the New Testament comes, not like the Old Testament which was brought to an end by the Lord, but as the one in which the Lord has given the fulfillment of the commandments, and after he had removed the old mysteries, he instituted the different mysteries of the revealed truth. Therefore, what he promised in the Old he has brought to fulfillment in the New. Since therefore the knowledge of the mysteries of the New Testament can be truly salutary and joyful only if one acknowledges that the promise that went before in the mysteries of the Old Testament is true, when the Scriptures says, "Do not leave an old friend because the new one will not be like him," there is immediately added, "A new friend is like new wine; when it has aged, you will drink it with pleasure." What does "aged" mean except that the type of the New Testament appears in the Old Testament? This new wine is thus drunk with joy if its meaning and promise are acknowledged in the Old Testament.