To everyone asking about the A7 instead of Amin7:
I'm not sure if what I'm going to say is true, but it's probably. Notice A is actually the V in the key of D which is the next chord in the sequence. You can alter the A7 chord too, since it's functioning! And, if I'm not mistaken, you can use D7 instead of Dmin7 too, because D is the V in the key of G, and probably altering it would work too (as would altering the G7 which is the V of C)
You can go all crazy around these sequences!
Please, bear in mind that whatever I say (or whatever real theory says) might be wrong. Let your ears be your judges!