Should one replace their silver fillings with the white ones. will it last just as long?

Yes and then No: Metal fillings have been the dentists' choice of restorations for many years. It is an amalgamated mixture that (once set) is very strong and will last for many years. Unfortunately, decay has been known to seep in between the filling and tooth structure causing cavities to develop under old amalgams. Tooth colored fillings are great looking but not nearly as strong. Still- i prefer composites.

metal fillings: Metal won't flex, so it can act like a wedge in the middle of your tooth, and since teeth flex during load, forces, etc. Your metal filling won't and long term can cause crack, fractures, open/creeping margins. Etc.

Won't last as long: Amalgam restorations have been know to last for decades, whereas white fillings can last 5 to 8 years. Regarding replacement, the choice is yours... Why not replace the silver fillings with white ones once the filling breaks down? This way you are not traumatizing so many teeth at one time.

That depends: If your metal fillings are in good condition, leave them alone. There is a chance your teeth could be damaged removing a perfectly good filling. If you have fillings that need to be replaced anyway then you can have them replaced with white fillings. They may however not last as long as metal fillings.

Amalgam vs.Composite: There are advantages and disadvantages with every type of filling. There is no one generally agreed upon 'ideal' filling material. It's best to have your own dentist help you determine when to replace your fillings and with what material. I use both, although i prefer tooth colored bonded composites in about 98% of the cases. However, i would not replace amalgam fillings just for the sake of it.