way back in the 1970's i believe the switch was made from deaf to hearing impaired in the height of mainstreaming where intensive hearing aid use in schools was widespread. I *think* back then deaf is equated with sign language in as so that as a mark of 'failure' on oral terms. So hearing-impaired became somewhat a shared terminology between audiologist and TOD's of this epoch. it is like they didnt believe people should be 'deaf' as too close to being language-less and /or dumb, the double d's - deaf and dumb. A kind of attempt to phase out these use of those terms. Indeed by mid-70s (1974 to be exact) an attempt to canonise a new term with a capital D - Deaf, was the first of the kind to play with words in attempt to define what IS deaf people all the while pulling the lid on the 'intra-deafs' conflict and struggle stemming from a form of elitism in which attached to the scale of ability in English, this is the language of the hearing, especially spoken English and even literancy in educational environments and outside. Humperies went on and added another term - Audism which stood for this.
Hearing impaired was a way of describing a potential of a deaf person to become 'hearing like' this description was widely encouraged as a 'new way to percieve' deaf people in so to consider they are not 'dumb', that is the word 'dumb' connects with the abstaince of spoken language ability. Again this has a looooong history, probably dates back to mid 16th century in one form , and another goes further back to Aristoles philosphising of humans and animals in relation to language. Aristoles was the first person to create an idea that deaf people were dumb, because they were lacking language. The sign language was aplenty even in those times, but all the while never understood it as a language per see. All this began to change with William Stoke 'made' the discovery of grammerical structures and rules in the 1960's, he was an English teacher- and hearing. Moreso when he discovered this, all the cohorts hearing AND deaf never believed that sign was a real language!! since even deaf people all this time for decades, centuries has been taught that sign is NOT a language !!
So, ok all I am saying I would think so strongly that hearing impaired as a term may have originated somewhere in the 1960's as a 'scientific term' and then used widely while administrating stern oralist mainstream programmes.
I REALLY hate that term "hearing imparied". I told my agency please remove that terms due to respect for the Deaf employees. 