No, the duct work does not have to be replaced when replacing a AC unit. However, if the ductwork is old or in poor condition, it may make sense to replace it at the same time. Replacing the AC unit itself is usually a fairly straightforward process, but replacing the ductwork can be more complex and time-consuming. If you are unsure about whether or not your ductwork needs to be replaced, its best to consult with a professional.