So far, we've seen truckloads of books on futility of German Z-plan of naval construction, but how about the U-boat program started in September 1939? It was clear for German planners that any war would have to won quickly and an U-boat ordered in 1939 would only execute patrols in 1941 by the time war would have to be practically decided. Meanwhile, training for a new, massive U-boat fleet would require massive effort, large number of experienced officers and smaller U-boats.
So, WI Germans end up with a conclusion that the most important effect U-boats can have in the war is to force British to do convoying, which reduces the efficiency of their merchant marine significantly. Thus the most important sub type to be constructed in accelerated fashion is Type IX, since it can force convoying even in distant waters. Thus it's ordered in OTL fashion. Meanwhile, the massive Type VII effort is abandoned.
What effects would this have? In 1939-1941 there would be more submarines available against the British as training effort would be smaller, thus more losses. Thus I would expect the British to spend all the resources, or even more, to U-boat defense as in OTL.
But the most important thing is, that U-boat construction effort required a massive amount of raw materials, steel, industrial effort and quality manpower. What would Germans be able to do with all the men and material wasted on Type VII's?