What do you do for a dog that is completely finished as defined by steady to wing, shot, fall, handles, and retrieves gently to hand reliably.
The dog returns to the owner with a clear understanding of how to protect their investment in said dog. I further demonstrate and provide a written list of "Dos and Don'ts". Finally, the dog returns to his/her owner.
Next spring the owner calls to say, "___ has become hard mouthed! What can we do?"
Finding a new owner is NOT an option.

Now as I see it, in Britain "rough" shooting is not as prevalent as driven shoots, generally. I presume that in a driven shooting setting it is rare to have high birds that are shot unusually close to the guns.
Locally, some rough shooting gunners are so bird-killing-driven that they either shoot far too soon OR shoot their semis (3rd shot) at multiple birds from a covey OR simply are not great shots and cripple an unusual number of birds. In-any-case, hard mouth can develop, post-training.
And so, I'm curious. What would your fix be, please? Keep in mind that the dog MUST return to it's owner.

For me, the fix has never been a cookie-cutter system and, can take a LONG time to resolve. Sadly, it doesn't take a long time to undo the resolve (one hunting season) because training the owner can be even more challenging.
I'm looking for a new/innovative/tried-and-true/tested/abstract or whatever you want to call it process/philosophy/observation....whatever?
Thank you ALL.