This is one of those situations where the narrative is fairly simple: people believed X, it was wrong, a few iconoclasts demonstrated in a fairly convincing way that Y was indeed the case, and then over time, the story got rewritten in the literature.

But in practice it turned out to be more complicated than the narrative; X is still somewhat true and ulcers and gastric disorders are both caused by, and affected by, multiple factors. Certainly, stress can exacerbate an existing latent ulcer or gastric disorder, making many people attribute the cause to stress, but this quickly gets into "proximal causes" and "ultimate causes".

As for working with doctors who didn't get the memo, I've learned that you sort of have to sneak up new information to them without making them feel challenged by a non-expert. It's a form of social engineering.