That's called an eggcorn.
acorn= looks like could be a combo of corn and an egg.
granite= cheap rock
take something for granite=take something for cheap rock
take something for granted= doesn't really paint a picture, but people prefer this one...........Slowly it will change to the alternate form. The eggcorn will become the accepted.
Characteristic of the eggcorn is that the new phrase makes sense on some level ('old-timer's disease' for 'Alzheimer's disease').
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