Depending on what changes you want, it may or may not be that easy to do. Scanned models are point clouds, and without expensive software, they are not easily "morphable". That said, it is easy to add to them by modelling up something and "adding" it to the point cloud.
I offer 3D scanning services. If you could send me some photos of the part you want scanned, and describe in more detail what you want done, I can get back to you with a quote and/or more detailed replies. You can e-mail me at firstname.lastname@example.org