The House modified the bill after it passed the Senate. Goldwater was among three Senators who voted against the bill. Can someone confirm that the modifications that the House of Representatives made were necessary for Goldwater to change from opposing to supporting the bill?
This shows he voted "Nay." This shows he voted "Yea."
I can only guess and assume that the modifications were why, but I have no other information than that. Sometimes Senators change their position as part of a deal with other Senators.