What would you call the following:
Speak now or forever hold your piece.
I would call this a malaprop, or malapropism. A famous one quoted on Wikipedia is from Yogi Berra, the king of the malaprops: "Texas has a lot of electrical votes" (should be "electoral" votes).
I've seen the term "homophone aphasia" used for this before, I think an eggcorn would be if you spelled the wrong homophone out of ignorance, homophone aphasia being more to do with you consciously knowing the difference and unconsciously making the mistake out of some sort of aging process, ie. "a senior moment".