Watch the Quick Change online guitar lesson by Brad Carlton from Chord Studies: Evolving Blues Vol. 1
When you have a "quick change" in a blues, it means that in measure 2 it goes to the IV chord. Which in this case, since we are in the key of A, is the D7. In this version we have also added the V7 chord in measure 12, which is a great way to make a progression want to start over, because a V chord always wants to resolve back to I.