From Longman Dictionary of Contemporary Englishchange sideschange sidesto leave one party, group etc and join an opposing party, group etc It’s quite rare for politicians to change sides. → change
Examples from the Corpus
change sides• And he is one of the few who have never changed sides.• Complete the set on one arm and then change sides.• Of course that didn't alter the fact that de Tracy had changed sides.• So many families had changed sides, and she was not imprisoned, nor even a slave.• Only a minority of the electorate changes sides between elections.• Angelina wondered whether she would change sides, just to show this pompous young man what she thought of him.• Release, then change sides to repeat with the other arm and leg. 