My recommendation would be to replace them as they start to leak or visually show they may fail soon. I have had that temptation when trying to fix Mercedes things and invariably you will find (especially often with a pro) that they will end up creating new problems due to their inexperience with the system that did not exist before. I know nobody would admit to it, but that happens frequently. Also, many newer products are not made as well as the old OEM ones. I have also had that happen numerous times. It will also be a lot harder to isolate individual problems if you just replace everything at once, but it is often a good deal for the guy selling the work.

Dkr.