This is a question people wonder about all the time.  They go to a dentist, and the dentist recommends one or more of their fillings be replaced.  And they wonder… “Gee… my fillings have not been hurting, they’ve not been bothering me, so why replace them?”

Let me tell you the “story” of the life of a filling.  This is a story that I’ve been telling for years.  I came up with it for the first time one day when I was doing a radio interview… and for some reason it’s stuck with me all these years (unlike funny jokes which I can never remember).
Your fillings “sprung to life” when (probably) as a child you got a cavity.  Your dentist at the time had to fix the cavity.  When cavities are fixed, the dentist drills a hole in your tooth (to remove the cavity).  Dentists can’t leave teeth with holes in them, so they “fill” the hole with a filling.
The filling’s only purpose is to fill up the hole, and seal it off so bacteria can’t get to the inside of the tooth.
Fillings usually last a long time, sometimes more than 10-20 years!  At some point in time, the filling looses its “seal”.  Just like a cork in a wine bottle.  When the cork is no longer sealing out the air, the wine spoils.  Similarly, when a filling is no longer sealed, a tiny, microscopic gap forms between the tooth and the filling.
Bacteria will then creep into that gap.  Meanwhile you, now an adult, are probably an awesome brusher and flosser.  You are keeping your teeth very clean, but no matter how well you brush and floss, you’ll never clean the bacteria out of those tiny gaps.
Well, as you may guess, those bacteria will begin eating away the tooth… only now they are eating away the tooth from the inside out.
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As these bacteria begin destroying the tooth from the inside out, the tooth will get very weak and begin to crack.
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Believe it or not, at this stage of the “life of a filling” there is usually NO PAIN or DISCOMFORT associated with the tooth.  
A person who is not proactive with their dental health will unknowingly allow the destruction to continue.  The new cavity will grow, the tooth will get weaker, and finally the tooth will break and/or become painful.  This is the stage where many people find out about their problem.  Only now what started out as a very small problem, has ballooned into a major dental problem.  
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The good news is that THIS PROCESS CAN BE AVOIDED.  If a person is seeing a proactive dentist, who will take the time to show their patient these problems when they are in their early stages, will avoid these major dental problems.
So my answer to the original question… “Should I replace my fillings?”…. is YES.  
Filling Upgrades are a great idea once the old filling is no longer effective.  
If you are interested in being proactive with your dental health, make sure your dentist is showing you close-up photos of your old fillings.