Switching REU recommender, good excuse?
Your recommendation letters should help you in any admissions program. They should speak to your strengths, but also be honest. If you feel that this professor will write something that can damage your application and you have the ability to switch out references/recommendations in your application without negatively impact your application, then absolutely, you should have another professor write you a recommendation letter. For the ones where you cannot switch, you should be honest in that you have another professor in mind who can write you a much stronger letter of recommendation - which is not a lie.