A gasket is designed to seal gaps between 2 surfaces. Depending on the material the gasket is made of, it may form a seal to the original surface, but not seal to a new one. You may get away with it, you may not, and I don't have any experience on the exhaust gasket.
If it was me, I'd get new ones and know the job will work properly first time. Better that then chancing it, finding it doesn't seal and needing the change them again later.