There was no decision to change it, they are two different terms. Global Warming is a subset of Climate Change. The confusion of terms exists only in the reporting of the general, non-scientific press and the minds of Internet dogs who think checking a household thermometer means they themselves are qualified to hold a valid opinion.
The IPCC was created back in 1988 at the request of WMO (World Meteorological Organization) and the UNEP (United Nations Environment Program).
The UNEP was formed in 1972 to study man's interaction with and impact on the environment.
The WMO was researching "potential global warming caused by the build-up of greenhouse gases in the atmosphere" back in the mid-1970s.
Back in 1956 scientist Gilbert Plass published a study titled "The Carbon Dioxide Theory of Climatic Change".
In a 1975 Science article by geochemist Wallace Broecker of Columbia University's Lamont-Doherty Geological Observatory: "Climatic Change: Are We on the Brink of a Pronounced Global Warming?"