TS - I think you set it up through Sprint. I'd imagine that you'd just have to tell them that you're planning on cancelling the ATT service. Only thing I'd seriously recommend is just taking baby steps with it (including repeating your intentions over and over, just so that the reps you speak with are clear on what you'll be doing). Last thing you want happening is for the Tier 1 reps to mess up and lose your number (happened to me on a Sprint account change/upgrade...wasn't even a port from 1 carrier to another...this was all before Dan Hesse took over though).
Love the G-Note series!