Someone JUST posted here today or yesterday (or was it a blog I read), with pictures, showing exactly how to remove quilting stitches on a longarm.
They folded back the top, with the quilt still on the frame, and used medical scissors - the kind used for removing stiches...duh! - and slowly cut a few stitches, releasing a small section. Then they moved on to the next few inches.
I've looked but can't locate it. Sorry. Maybe someone else will have better luck. But I'm sure it was in the last 24-36 hours.