Historical fiction, simply defined, is when an author puts fictional characters against a setting of real historical events. Usually, to call it historical, the fictional characters need to interact with real historical figures or take part in/be impacted by the actual events.
What defines a historical event? Some people would call a novel set in the 70s historical fiction. Personally, I'm offended by that. I remember the 70s. Sort of. The 1950s is borderline. By the time you get to WWII, it would definitely be classified as historical.
Just because you can take a plot from one era and make it work just as well in another era doesn't mean it isn't a historical novel. However, the more the story depends upon the actual historical event, the more truly "historical" it is.