Back to the question at hand, what really matters here is whether the new program you're interested in uses the match to fill most of their spots, or not.
There are two possible scenarios here. In the first, Program A doesn't use the match, ever. Program B does. You interviewed at both, and then matched at program B. Now program A has an open off-cycle spot (i.e. starting in July 2012,rather than this coming match which is for July 2013 Advanced).
In the second, both Program A and Program B use the match. You applied to both. You matched in B for July 2012. Now, Program A has a spot open for July 2012, which is "not in the match" because this year's match is for July 2013 Advanced.
If you are talking about scenario 1, then theoretically you could switch programs. You'd resign your spot in Program B. It would be a match violation. You would be banned from the match for 5 years. You'd join Program B. You'd finish training. Done. Win. Or, as above, medicine is a small world and there is some chance that the PD of Program B would get really angry, call the PD of Program A, and you might find that your whole career comes to a crashing halt. That would be bad, and you would not be able to match into anything.
If you're talking about scenario #2, then you cannot do this at all. That's because, not only would you be violating the match, but so would Program A. And they will be withdrawn from the match, and they will not like that.
Can it be done in a "safe" way? Perhaps. If you get both PD's to agree, then you might be able to do it "under the radar". You simply go to Program A, and never tell the NRMP anything. Or, you could ask the NRMP for a waiver with support of both PD's, but they have become very stringent recently -- and I can understand why. Imagine you come to me, and say "Sorry dude, I really want to go to this cool new open spot rather than your program". What exactly am I supposed to do? I could be a jerk and tell you that you have to honor your match commitment, and then you'd probably be bitter and angry at me. Or I could be a wimp and let you go,leaving me with an open spot and trying to find someone good to fill it -- which usually goes unfilled, or I take someone significantly less competitive, or I try to "steal" someone from another program and simply pass the problem off to them. So, the NRMP plays the "bad cop" and enforces the rules.