I've never personally seen someone change their vote in the game before, so I'd need to actually look it up somewhere to find out. If anyone (who actually has experience playing this game or knows the rules) wants to let me know before I look it up, they're welcome to.

Edit: Looked into it a bit, and from what I can tell, you're all free to discuss who you think it might be, but once you make an official vote for someone then that's it until the next round.