The ceramic collar alone cannot be replaced, but the socket can be replaced. They come with two wires that need to be connected in place of the old socket. Without seeing your fixture, though, I can't tell if it would require a special bracket in order to mount it. I actually have a spare socket with bracket, but again, no way of knowing whether it could be easily mounted in your fixture.

You might want to contact the fixture's manufacturer if you're not sure what you need.

ramsay,

If you are referring to the ceramic shell around the metal screw-in base then that IS the MOGUL socket that all HID lamps use. The ceramic MOGUL socket is absolutely standard (the screw holes will match), and is available at all elecrical supply stores for under $10. If it is some part beyond the basic socket then I don't know what that is. I pretty much can get all of my MOGUL sockets from knocked-down streetlamps, thanks to those with lesser driving skills. Hey, it's my tax dollars laying there on the ground.

Zink