Change-Up (1934)

“The old Maine Moose Hunter [Clarence Blethen] was mixing them up, pulling the string now and then for a perfect change-up and then blowing that old fast one and curve right in there to catch the Crackers flat.” Jimmy Jones, The Atlanta Constitution, April 27, 1934, p23

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1948. “He’s got everything—speed, curve, change-up and plenty of heart” (Birmingham News, May 7, p. 46).