do an about-face
Also, do a flip-flop or one-eighty. Reverse one's opinion or course of action. For example, The board did an about-face on acquiring more land, or We expected Dad to do a flip-flop concerning our vacation plans, or They had relied on Jim to vote for Harry, but he did a one-eighty and cast his vote for the incumbent. The first term, alluding to the army command to turn around, dates from the first half of the 1900s, and the variants from the second half of the 1900s (the last refers to a 180-degree change of direction).
Dictionary.com presents 366 FAQs, incorporating some of the frequently asked questions from the past with newer queries.