i'm almost the opposite to you! If it's low-rise, I have to wear larger size than if it's "just below"...

and about the body changing shape... i don't think the "basic shape" changes... everything will *eventually* go proportionally... I too lose faster in my upper body than the lower part... but and when I'm losing weight, my pant size doesn't change. Only my arms (and unfortunately, the other upper part areas) get smaller. Only when my weight stabilize for a bit that my pant size begins to drop. I guess that's when the fat migrates north... (and then it always seems like my arms get a bit bigger as a result...)