Your rail/upright and top plate do not carry any wight, they just stiffen up the structure, you can leave your pool full and remove all the uprights and top plates without adverse effects - however, frost heaves saturated soil drastically, so if the soil there has a tendency to get very wet, if frost comes, your wall WILL push up in that area, this causes all sorts of repercussions you don't want to deal with, like the liner pulling off the top, the wall buckling and probably many other things.
You may want to put proper drainage before building the pool (like a ditch filled with big o tube and gravel draining to a lower part of the yard).