Modern submarines without deck guns in the 1939 war

What happens if Germany replaces its 1939 U-boat fleet with modern submarines that it can repair but cannot replace?

They would be quieter and more capable, but their lack of deck guns would force them to spend torpedoes on every kill.

The replacement submarines would gain substantial advantages in quietness and capability. They would also lose the deck guns still useful in the early-war environment. Relying entirely on torpedoes would exhaust their ammunition more quickly. The answer distinguishes the needs of the 1939 campaign from those of modern operations under a much greater aviation threat.