Take the area of intellectual deficiency. As knowledge of this condition increased, and possible means of intervention were being developed, it became useful to distinguish different degrees of deficiency. Hence, moron, imbecile, and idiot.
At one time, the terms "moron," "imbecile," and "idiot" had specific meaning. Morons had a mental age between 8 and 12 or an IQ in the 51-70), imbeciles had IQs in the 25-50 range, and idiots had IQs below 25. Because those terms came to be used loosely as insults or other non-diagnostic usage, in the 1960s the American "Asociation on Mental Deficiency adopted a four-category classification system:
Mild retardation IQs between 50 and 69
Moderate retardation IQs between 35 and 49
Severe retardation IQs between 20 and 34
Profound retardation IQs below 20
Older terminology used to refer to specific syndromes also changed: cretinism became infantile hypothyroidism and mongolism became Down's syndrome.
Indeed, the term used collectively to refer to this type of disability has changed. Once, it was known as "feeblemindedness"; but mental retardation was substituted as an euphemism. However, in recent years, there was been a push to substitute "intellectual disability" for mental retardation. In 2010, President Obama signed Senate Bill 2781, which changed references in Federal statutes from mental retadation to intellectual disability.
My guess is that people, in their mania to dysphemize, will eventually turn intellectual disability into an insult. Politicians, the media, academics, and good old-fashioned perverseness will provide the fuel for that linguistic change.