Well... High Walls do upgrade Walls, which actually replaces wall erasing them from the list of city buildings (I checked in WB cannot have both at the same time, heaving HW automatically undo walls), but High Walls should be good enough as prereq. for The Ishtar Gate.
If upgrade cancels prereq for building ...well maybe it should carry abilities of a replaced/upgraded building as a prereq? I am sure there are more cases like this one.
Edit: What about adding building upgrade tree as eventual list of buildings needed to get a wonder.... does it use on "and/or" operators or only "and" ? (sort of :wall or high walls or ....)
Edit2: The "legal" way without WB is to sell HW ...than build walls again ....