Certainly more is implied than a single substitutionary exchange. Because the text reads χάριν . . . χάριτος (and not, e.g., χάριν . . . ἀντὶ νόμου), the reference is to “one blessing taking the place of another in succession” (Regard 68), to replenished grace, to a rapid and perpetual succession of blessings, as though there were no interval between the arrival of one blessing and the receipt of the next. “God’s favor comes in ever new streams” (BDAG 88a), or as Robertson puts it, “As the days come and go a new supply takes the place of the grace already bestowed as wave follows wave upon the shore” (574). The nature of the constantly renewed grace remains undefined but probably refers to the multiplied spiritual benefits of the new covenant (e.g., 1:17). 

Murray J. Harris, Exegetical Guide to the Greek New Testament: John, 38.