I only played the war wagon once - at my first WM game ever, so I don't know if there is a real need for it to be amended.
If so, then I like Aldhick's proposal of the two stands acting as one (meaning if the horse stand comes into contact with enemy, it counts the same as if it were the wagon stand.)
The other way (of moving the unit backwards) definitely would look silly. And it would mean you get -1 attack for having an enemy on your rear. (If I did not miss anything - I'm not looking at the wagon rules at the moment.)