Unless necessary it is not required to replace all fillings. Necessary means presence of cavity, possible leakage which could cause cavity, sensitivity, discoloration and esthetic according to patients feeling regarding discoloration, cracked, broken and such. If you’re front restorations changing color you may want to use hydrogen peroxide for human consumption ( health stores have it) at night after eating and brushing. Maintenance is better than replacement, every replacement requires further tooth structure removal and additional issues in future. Keep in mind I am also very conservative. Replacing a filling may only require extremly little reduction of the previous restoration not the entire filling, the back can always be sanded and polished.