Here is something interesting I read on a natural foods forum.

A woman says that she had fillings put in 45 years ago and wanted to get them out. But first she consulted with 2 chiropracters who did muscle testing on her. They told her that it could be harmful to have such old ones removed. Not clear why. But in any case, most of the mercury is gone into the body and she would just be wasting her money even if the removal didn't do any harm. So she is detoxing instead.

I'm just telling you what this person said. I have no opinion either way. I have no particular faith in muscle testing. Maybe other kinds of tests should be done before one decides a filling is too old to stay in, I don't know.