There is also a trick very similar to the above method that longarmers use when a quilt has a problem. We starch the area heavily, and then steam the area that's off with the iron. Sometimes you may need to repeat. This will pull in any areas that are out of whack (like the border areas that are too large). Once I got it back into square, then I'd add my borders on, keeping the squareness. It's called the Starch and Steam Method and works like a charm.
We can take a quilt that is out of square, or has 'D' cups in the middle, and pull it back in to make it lie flat for quilting. It takes a bit of time, and sometimes you will have to repeat it, but it's easier than taking it apart. If you decide to try it, and have questions, please don't hesitate to ask.
Hope this helps.