The Encyclopædia Britannica claims the witnesses denied their testimony later in life.
The Encyclopædia Britannica, 9th ed. (New York, NY: Charles Scribner's Sons, 1883), 16:826
Being himself unable to read or write fluently, Smith employed an amanuensis one Oliver Cowder, to whom, from behind a curtain, he dictated a translation, which, with the aid of a farmer, Martin Harris, who had more money than wit, was printed and published in 1830 under the title of The Book of Mormon, and accompanied by the sworn statement of Oliver Cowdery, David Whitmer, and Martin Harris that an angel of God had shown them the plates of which the book was a translation. This testimony all three, on renouncing Mormonism some years later, denounced as false; but meanwhile it helped Smith to impose on the credulous, particularly in the absence of the gold plates themselves, which suddenly and mysteriously disappeared.