hi there,
i saw this post earlier and figured i'd give my two cents if no one else took interest in replying.
i was in a similar predicament a few days after april 15th. I committed by the April 15th deadline, had second thoughts as I had another school I was considering, but ultimately decided I was happy with the choice I made and stayed with it.
An important factor in all of this is whether you received some sort of funding. I don't think the cgs (what's apa?) resolution is a binding contract, but to renege this late in the process could potentially complicate things for that department. They may lose that funding or they may not find a replacement at this late stage.
I understand after making a big decision like this you are met with new waves of doubt and regret, but try to have some resolve. i was definitely critical of myself in the days after thinking what could have been, but after all, we are talking about graduate studies (and paid for at that) which is a privelege rather than a right. Be happy with that, either place you will probably be given opportunities not many others have.
But don't make it a big moral question. If you are truly so against where you're currently committed to go to school, then just pull the plug. But act quickly. It may not be too late for someone else to receive your spot.
In my personal opinion, I recommend you stick with your original choice to achieve a certain level of integrity. I was motivated to stick with my original choice for many reasons (small, friendly program/good funds/nice environment) but maintaining my professional honesty definitely meant something to me as well. That is not to say you should keep commitments only because of some sort of binding agreement you don't want to break. If you can't see yourself being there for the long haul, then you are really doing yourself a favor as well as the department by backing out before too much money's been spent.