Dead as a doorknob: idiom changes due to misuse

Most (if not all) of us have likely heard the phrase "dead as a door-nail."

However, I have noticed that a large portion (ok, all) of my American university students of the last 5 years erroneously think that this phrase is instead "dead as a doorknob."

I assume this might be due to the relatively infrequent use of the word/phrase door-nail in common speech, especially when compared to "doorknob" (e.g., see here, here, or here).

• In fact, most of my students are unsure what a door-nail is (though many can guess).

I assume that if my students represent a broader trend, there is a chance that the idiom might actually permanently "change" from using doornail to doorknob in the not so distant future.

My question: Is there a word or phrase to describe an instance in which an idiom actually changes due to a change in common (mis)usage?

A malapropism is

the usually unintentionally humorous misuse or distortion of a word or phrase; especially : the use of a word sounding somewhat like the one intended but ludicrously wrong in the context. “Jesus healing those leopards” is an example of malapropism.

Perhaps the OP is searching for the malapropism of a word within an idiom, such as "for all intensive purposes" or "it's a mute point." (Should be "for all intents and purposes" and "it's a moot point.")