That would only show that people are ignorant of the past and influenced by what is called the national myth [0], not necessarily that their definition of fascism has changed.
To return your argument, if you poll people who call current events fascists and ask them if that 1923 Supreme court decision is fascist, would you be as confident that 95% would say it's not?
That people's view of fascism has changed after world war 2 is obvious and not particularity insightful. So has their view of antisemitism for instance.
I also should add that I agree with you on the great danger of labeling too many things fascists, including the current events. It is entirely possible to oppose Trump's second term and even think that it is a threat to democracy without resorting to calling it fascist. It is also possible to compare it with the rise of fascist regimes if one provides appropriate arguments.
I just don't think that your example with 1920s America illustrates that point particularly well.
[0] https://en.m.wikipedia.org/wiki/National_myth
