Philosophers of science have recently called for experts-by-experience to be involved in revising psychiatric diagnoses. They argue that experts-by-experience can have relevant knowledge which is important for considering potential modifications to psychiatric diagnoses. I show how altering one diagnosis can impact individuals with a different diagnosis. For example, altering autism can impact individuals diagnosed with Attention Deficit Hyperactivity Disorder and Schizoid Personality Disorder through co-morbidity and differential diagnostic criteria. Altering autism can impact the population making up the diagnosis of Attention Deficit Hyperactivity Disorder and Schizoid Personality Disorder which can then influence which behaviour each diagnosis covers and which parts of the causal structure each diagnosis covers. Expanding one diagnosis can mean, if differential diagnostic criteria are present, that individuals who have a different diagnosis, or would one day have been diagnosed with that different diagnosis, are instead now diagnosed with the expanded diagnosis. Alternatively, if two diagnoses can be co-morbid then expanding one diagnosis can mean individuals who have a different diagnosis can now also receive the expanded diagnosis. Changing these can then impact the adequacy of symptoms formulated to cover behaviour and causal mechanisms formulated to cover the causal structure. This means experts-by-experience representing a range of diagnoses should be involved in decisions over modifying diagnoses they do not have. For example, Attention Deficit Hyperactivity Disorder individuals should be involved in decisions to modify autism. I discuss practical issues relating to how much say people with a range of diagnoses should have in relation to modifying a diagnosis which they do not have. We should employ a principle of equality whereby the level of say individuals with one diagnosis should have on another diagnosis should be equally reciprocated.