Taking out an amalgam filling and rebuilding the tooth in a tooth-colored material.
The old filling is removed, the tooth underneath is checked, and the space is rebuilt in composite.
There is a reason we say sensitivity is more likely here. An amalgam filling that has been in place for twenty years sits in a tooth that has adapted around it, and removing it disturbs more than a first filling does.
Sometimes the tooth underneath turns out to have less structure left than the x-ray suggested, and a filling will not hold. We stop at that point and talk about a crown rather than carrying on.
Not because it is silver. A sound amalgam filling can stay where it is for decades. We replace one when it has broken down, when decay has started underneath it, or when you have decided you want it gone.
This is informational; consult your dentist for diagnosis and treatment.