You then have to make a decision
about whether to replace the whole exhaust or just parts of it. Often, if one part is corroded, it is likely that parts of a similar age could be corroded too. Sometimes the different parts of an exhaust fuse together because of heat and they are starting to corrode. That makes separation of the individual parts extremely difficult.
But when they haven't fused together, it's a matter of judgement and consideration based on vehicle usage, driving habits and distances travelled, that will dictate whether individual parts should be replaced or the entire exhaust assembly. Sometimes it can save you time and money to replace more than the corroded part