Some people wonder if their lips will change after genioplasty, and the short answer is yes, this will be a tradeoff.
A genioplasty, especially one that moves the chin forward will make lips appear thinner, especially the lower lip by curling it slightly inward. This is due to the pull of the soft tissue by the advanced bone. Also the mentalis muscle that controls the lower lip is cut and needs to be reattached after the surgery. How precisely and where it's re-attached will effect the lower lips appearance.
Further Reading
Evaluation of Lip changes and Smile Aesthetics following Genioplasty: A Retrospective Study