They didn't all become Republicans. This is evident from the 1964 Civil Rights Act-
“Of the 420 (House) members who voted (on H.R 7152), 290 supported the civil rights bill and 130 opposed it. Republicans favored the bill 138 to 34; Democrats supported it 152-96. It is interesting to note that Democrats from northern states voted overwhelmingly for the bill, 141 to 4, while Democrats from southern states voted overwhelmingly against the bill, 92 to 11.”
I added the italics. The quote comes from this link
http://www.congresslink.org/civil/essay.html
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