I need to replace the outdoor unit of my AC or heat pump, but the indoor unit is fine. Should I replace the indoor part too?

Usually, this is a good idea during air conditioning replacement, unless financial circumstances make it impossible to do so. When it comes to air conditioners and heat pumps, the indoor and outdoor unit are designed to work perfectly together. Although a new outdoor unit can run with an indoor unit it wasn’t designed for, it won’t run as efficiently or work as effectively. If possible, you should probably replace both units at the same time.