According to this AOPA article, the change was made in Sept, 1993
In a move to simplify airspace classification, the FAA will rename the different airspace categories on Sept. 16, 1993. In order to fall in line with ICAO (International Civil Aviation Organization) practices, it will replace descriptive titles, such as Terminal Control Area, with alphabetical designators, such as Class B Airspace. This change will be phased in gradually. Charts and references will carry both designations until pilots get used to the transition.
And some more background
A Brief History of Aeronautical Charting
To understand the change that GIS has brought to the National Geospatial-Intelligence Agency (NGA), it is helpful to be familiar with the history of aeronautical charting.
Shortly after the Wright brothers made their historic first flights, the skies began to fill with aircraft. Visibility was the key navigational tool at that time. Aircraft were limited to short flights in clear weather and used transportation routes to navigate by, flying low to the railroads during reduced visibility. Early pilots began making personal notes to help them navigate to and land at increasingly distant airports, and enterprising pilots sold these notes to other pilots, but air travel remained limited by visibility.
In the 1930s, radio technology made it possible for pilots to navigate farther distances through unfamiliar surroundings in reduced visibility. In 1941, the first instrument approach and landing charts were developed, serving pilots with the need to land in low visibility.
By this time, aviation was a matter of interest worldwide. Many organizations began drafting standards for aviation-related maps, charts, and information. During World War II, the demand for charts increased dramatically. By 1943, production had increased from around 500,000 per year to more than 11 million. The U.S. Army and Navy air forces each built their own custom charts to fit their wartime needs.
By the end of the war, it was clear that standardized products and symbology were needed to support international air travel. The following years saw the establishment and maturity of many of the aeronautical- and aviation-related agencies and associations that we see today, such as the U.S. Federal Aviation Administration and the United Nations International Civil Aviation Organization (ICAO), each mandated to ensure the safe, efficient, and orderly evolution of international civil aviation.
Today, these organizations and others drive the look and feel of aviation products worldwide. They have also mandated an update cycle to ensure that all aircraft are flying on the same data. Depending on the region of the world, this cycle is effective every 28 days or multiples of 28 days. It is easy to see how a map of the world's airports and airways, updated every 28 days and limited by international standards, can become a huge challenge. This challenge is the mission of NGA's Aeronautical Division.