Confirmation bias is an existing concept and may be a driver for the conviction but is not what’s happening here. It’s more akin to someone coming out and saying “I used to be a strong supporter for the democrats and voted for Hillary in the election, but the covid pandemic has made me a firm Trump supporter due to his beautiful handling of it!” And then someone finds a series of Tweets proving the person was openly supporting Trump during the election.

I would more refer to it as a false flag conversion. Pretend to be the enemy and come out with a fake story about how some compelling argument “forced you” to convert to the side that really is your true colors.