I have been wondering this for a while and don't think I could find a definitive answer. When WWII began in 1939, what did they call it? Because as all of you know the WWI before was called the Great War not WWI at the time, so what changes were made to call that war WWII and then give The Great War a new name, WWI?
Variations on this question are frequently asked in this sub, which I have addressed here at length, with links to previous answers that address the specifics of your question.
TL;DR: It's something of a myth that the First World War was ever exclusively called "the Great War". "World War" was used as an alternative name from that war's earliest days. By the war's end in 1918, "World War" was the more popular term in the United States. In 1919, "World War" became the official name for the conflict by the U.S. War Department in all official communications. The press followed suit, and "Great War" rapidly fell out of favor.
"The Great War" lasted longer in the U.K., but both terms were well understood by the time the war was over, and the terms remained interchangeable throughout the interwar period. So when the events that led up to WWII threatened to lead to widespread war, the press began making predictions of a "second world war". Within days of Germany invading Poland in September 1939, the conflict was coined by the English-language press as "the Second World War" and "World War II" and the name stuck. Some politicians tried coining alternate terminology, but nothing else ever caught on.