The RN changed all the Fleet codes simultaneously in August 1940, after becoming aware of German successes in reading the codes..
Germans then had trouble reading the Fleet codes, but could still read about 50% of traffic but with a greater time lag than before.
The Germans never read the RNs high level book codes.
In September 1941, the indicators in British naval codes ceased to be encyphed.*
This improvement on its face, as far as the Germans were concerned, did NOT translate into better operational results at sea. SURPRISE!
British SI now held the upper hand.
* NB This allowed false data/disinformation to be fed to the Germans , sustaining what they believed and what the British wanted them to believe. Sneaky Brits.