The damage to the Germans from Enigma decrypts was so enormous that it would be far too expensive for Germany not to revamp its crypto.
Any deception operation based on sending false information on known broken keys would require continuing use of Enigma for all real traffic as well. Otherwise the Allies would instantly realize that the Germans had discovered the break.
The only remotely comparable sacrifice of this sort that I know of occurred during the Cold War in Berlin. American and British intelligence figured out that a phone line used between two KGB offices in East Berlin ran underground near the border with the western zone.
The US and UK spooks dug a tunnel under the border to the cable run, and tapped it into it. (IIRC, the taps were installed by British technicians and monitored by US listeners.) The results were useful, but not explosive.
As it happened, one of the British operatives was George Blake, a Soviet mole, so the Soviets knew all about the operation. But if they abruptly shut down use of that phone line, that would reveal the existence of a mole and possibly "blow" Blake. So KGB headquarters never told their Berlin people about the tap!
Anyway, it's very important to determine when the Germans realize Enigma is broken. It could happen in 1940. It's not widely known, but the break into Enigma in early 1940 was a joint operation of GCCS at Bletchley Park in Britain and a group of French and Polish exile cryptanalysts working for the French Deuxiéme Bureau at PC Bruno, SE of Paris. The two groups shared all deductions about Enigma. From March 1940 until June, they each decrypted a few thousand Enigma messages. (At this time, neither country had the processes in place to use Enigma effectively; the main effect was to reveal to British leaders the scope of German victory, and that it would be useless to send any additional forces.)
PC Bruno was evacuated, with the Poles going to Algeria. They were later brought back to unoccupied France, and resumed breaking Enigma until after TORCH. This work was still under "official" French intelligence; that is, the Vichy government. (But it it does not appear that the French spooks ever told Vichy civilian leaders about it; nor even Darlan, who was in charge of reorganizing French intelligence after the armistice.)
The leak could happen if the evacuation of PC Bruno wasn't "clean"; that is, if papers were left behind exposing the work, or some of the personnel didn't get away in time. Or some official "in the know" might tell the Germans - for money or other rewards, or perhaps anger towards Britain after Operation CATAPULT, which sank French ships and killed over a thousand French sailors. (Perhaps including the key man's only son?)
The effect would not be in damage the Germans could do to the Allies, but in damage the Allies would not do to the Germans. Most of the intelligence from Enigma was about the conditions and strength of German forces. It made Allied commanders' jobs much easier knowing almost as well as the Germans what the Germans had.
The great exception was in naval activity, where Enigma messages often were operational orders directing ships and submarines to specific locations. Bletchley Park started reading German navy Enigma in mid-1941. This allowed the Admiralty to redirect convoys around U-boat scouting lines.
The German tactic was to spread out a group of U-boats. When a convoy position was reported to HQ in France, HQ ordered the U-boats (faster than a convoy) to gather ahead. They would attack in mass at night ("wolfpack"). But when the British were reading the orders, they could "play blind-man's-bluff with their eyes open". Allied shipping losses dropped by over half until early 1942, when the Germans introduced a new Enigma variant for U-boats, and dropped again after Alan Turing broke the new key in November.
ULTRA was also tactically important in the Mediterranean; it told the Allies where to find Axis ships running between Italy and North Africa.
Finally, ULTRA included messages between Abwehr HQ in Germany and its outstation in Spain, which "ran" most of the purported German agents in Britain. These were actually double agents under British control. By reading German internal discussions about these agents, the British could "tune" their performances to be more convincing.
ULTRA was enormously valuable, but exactly how valuable is not easy to say.
