Blender Mirror Help

I edited the shoe shape in the 310.n2km file... Then I created a duplicate of the left shoe (the one I edited) and mirrored it to the right.. Deleted the original right shoe and renamed the mirrored one to the name of the deleted one and exported. But when I import it again, the mirror resets and there are 2 left shoes again and no right shoe (because the other one is supposed to be mirrored). What am I doing wrong?

from the sounds of it, nothing, but blender and 3D editing the 2K files are finicky. Best bet is to redo the edits to did to the left foot to the right foot.