Thanks blairrorani. I tried dragging the layers like this yesterday (seemed like the most obvious thing), but couldn't get them to move at all. But after looking at this video, I just tried it again, and it worked! So thanks for the info.
I wonder if my inability to move the layers had something to do with the pen connection? I found when my Adonit Jot Touch was connected I had trouble with the long touch fill as well. When I just now successfully reordered the layers, I did not have my pen connected.
It seems like you have to hold your finger or stylus/pen on the layer for a second, then the layer is moveable. You can see a slight difference in appearance when it's ready to be moved.