In general you should replace both the outdoor and indoor sections of your AC system during a replacement. Current AC systems are rated as "matched systems." This means that in order to operate a peak efficiency, the indoor unit and the outdoor unit need to be design to work together. If you have an older existing AC system, your indoor coil is definitely design for a less efficient system, and will not provided appropriate efficiency with a new outdoor unit. Your older system my be design for older R22 refrigerant. R22 coils are not appropriate form use with modern R410A systems.