How hard is it to have a metal filling replaced with ceramic material? is this suggested, or is it best to leave the filling in as is?

Ceramic for metal: It is very easy to remove metal fillings for ceramic. It the filling is old and breaking down it is best to replace it. Ceramic restorations are also bonded to the tooth and help strengthening the tooth. If the filling is very small it can also be replaced with a small resin. A ceramic filling can usually be put in the same day with cad/cam technology.

Not very hard: Its harder to give a complete answer because of not knowing why you asked as well as the type of material you are talking about. If there is nothing wrong with the filling leave as is. Are you referring to an amalgam or gold inlay? By ceramic are you referring to composite or a porcelain type like a crown that a dental lab makes? If a real large filling, a partial or full ceramic crown is option.

Not suggested: If you have no symptoms, leave the fillings as they are. When they 'need' to be replaced, that is the time to place 'white' fillings instead.