If you want to change it from one type of mount to another you get an adapter. You generally don't replace mounts unless you are trying to repair a lens. There are all kinds of adapters now on E bay. I bought some to convert my old manual lenses to working on my new DSLR. As far as a ring, well I have no idea what you might mean unless I could see it.I don't know about new lenses but in old ones there were spacer rings that were put in to adjust the focal point of the lens. In order for a lens to be focused at infinity when it comes to the infinity mark and stops it has to be adjusted to the correct distance and this was done with spacer rings. If the spacer is off the lens will not be able to reach its infinity focus or it will go past it and need to be adjusted back. This is something that is done in the factory where they have special optical equipment for getting the settings just right. By trial and error its pretty difficult.