Ferendial -- a few other posts were added while I was writing my first response to this thread, so I've read a little more about how your circumstances came about. If bids were to be given out at 10 pm, I too would have expected a phone call before 10, not after. It sounds like the colony chapter dropped the ball on that.
It does sound like you accepted the bid with good intentions, and your reasons for preferring to depledge sooner instead of later are understandable. The fact that you are seeking guidance and still considering remaining in your fraternity --rather than just up and pulling out -- indicates to me that you do want to manage this in the best way possible.
I don't know what the policies of IFC and most fraternities are on this, but there are Greek Chat members who are very knowledgeable about it. They should be able to offer you wise counsel. I feel like you will probably be advised to remain in your fraternity and there are many at GC who can give you solid advice on how to go forward.
Whichever road you take, I wish you the very best!