That's easy enough to say, but I wouldn't make it a general rule too quickly especially with larger fillings that just need a small repair. We evaluate restorations every day to determine whether they need replacement or not so I don't consider this that much different.
I personally would weigh the risks and benefits of each case and carefully consider the potential of iatrogenically creating an irreversible pulpitis when it could have been avoided. Consider the consequences of making a problem when there wasn't one to begin with. I would much rather inherit someone else's problem than create my own.
Here's a scenario for you. Patient comes in, notices just a small cavity at the buccal margin of a large filling. Tooth has never bothered him for 15 years. You check out the filling, and all margins except that buccal are intact. No radiolucencies on the xray. You say, let's replace the whole thing because I don't know what's under it. I don't want to inherit some other dentist's problem. Patient trusts you and says okay. After placing the new filling, the patient has a severe lingering ache and requires a root canal due to removing and replacing the large filling that was very close to the nerve.
Imagine a similar scenario : you or your mom or litigation lawyer has an old deep amalgam filling that looks like it is sitting 0.5 mm above a pulp horn with no symptoms, xray findings or pathology present for the last 15 years. It has a small marginal defect which is decaying but otherwise is fine. Would you ask the dentist to replace the entire filling when he tells you that he thinks that a repair would take care of it?
If you're comfortable in dealing with the consequences and have thought over your choices carefully that's fine, continue with your preferences. To each their own as they say. If you are just following something that some other dentist has told you in passing during a clinic or somewhere, I suggest that you think over the pros and cons of that advice.
