We used to get a lot that were broken when they went into a chipping machine (well, actually they were chewed/broken) and the centre is made up of 2 halves (glued together I think) and they have a groove which often had material (plastic ring) inside so it's possible. I know that if they came out, it was sometimes possible to pop them back in so I don't expect it's too much hard work if determined enough. It was a pain in the neck as they used to jam the chipping machine so you'd have to "pause" it and try to receive the centres (and any broken outer pieces) before the machine would work properly again. I have a Victoria £100 centre that shows the groove quite well.