This is a myth!
Some people think that even if you stick to your plan, you’ll gain weight during yom tov. The reason for this worry is because it’s a “change in diet”. I put that in quotation marks because it really isn’t. You’re simply substituting SOME things for other things for a week and a half. You’re still eating within the guidelines of your plan regardless of the temporary changes in foods. Also, it’s not for a few months where your body adapts, but rather a temporary change from which you can easily switch back. Stay strong, stick to your plan and have a HEALTHY Pesach!