If you play a theater of war mission right now, the army you use is called Wehrmacht.
It has been this way for as long as I can recall (try operation barbarossa for example).
It's not a name change as far as I can tell - just its never been properly made clear.
I find it easier to think of them as the same army, at different points in time. The OKW are clearly 1945, with top tier technology like IR scopes and rocket weaponry, whereas the Wehrmacht are the general army that fight on both fronts (hence always grenadiers/panzer grenadiers/half tracks/pz4s etc)
