I unscrewed the cap on the header tank, to let air in...at first I couldn't figure out why it wasn't draining...duh!
Also, I noticed that not all the coolant flowed out --only about half of it did...which I didn't realize until later when I was filling up the new stuff. I suppose I should have ran the engine a bit to help pump most of the rest of the old stuff out --I assume it's normal for it not all to flow out via gravity when you first open the drain, since a lot of the coolant is in the passages in the block and cylinder head?