If you're using UE4 you can do what Eric suggested by simply selecting a mesh in the content browser (that you want) and the one in the level(that you want to replace), clicking the arrow under Static Mesh in the details panel, and swapping out any static mesh for another.
I would simply snap the solid wall to the windowed one, then delete the windowed one. I use Layers to help me organize assets like this. I would put the windowed walls in a different layer from the solid walls. Makes it easier to select/hide/freeze overlapping assets.