“Of His fulness we have all received, and grace in place of grace. For Law came through Moses; grace and truth through Jesus Christ.”
In the closing sentences of his introduction, John lays out a contrast between Moses and Jesus. Moses gave us one thing, and Jesus is now giving us something in its place. What we got through Moses was the Law. What we get through Jesus is grace and truth.
If that’s all he said, then we could walk away with a simple contrast: law on one hand, grace and truth on the other. But it’s not that simple, because John tells us the substitution is “grace in place of grace.” It’s not law vs. grace. It’s the Law-that-was-grace-already vs. grace-and-truth-through-Christ.
We had one sort of grace; we are now being given a different, a higher, sort of grace. There’s a real contrast here, and John wants us to feel the difference. He also wants us to know that contrast takes place within a continuity of divine grace toward us. Jesus changes everything, and yet Jesus is in keeping with everything that has come before.
When you read what comes before, read it with this in mind.
Full Contact Christianity 
Amen. Precisely where I landed on this text.