Or HaChaim on Leviticus 26:10/אור החיים על ויקרא כ״ו:י׳

1 ואכלתם ישן נושן, "And you will eat old store, long kept." This is a promise that the harvest which has been stored will not rot or become worm-eaten. On the contrary, you will experience that it improves with age.

2וישן מפני חדש תוציאו, "and you will bring forth the old from before the new." You will not bring it forth because it has become too old and inferior, but because you have to make room for the new harvest. The reason the Torah wrote this here and not earlier when the goodness of the land of Israel is extolled is that the Torah wants to make the point that although the population keeps increasing this does not mean that there will be a contraction of the surplus experienced previously. There will still be so much that the people will eat the old rather than to have to eat the new harvest right away.