While you may be able to get away with just replacing the outside AC unit, it's usually not recommended. Replacing the outdoor and indoor units at the same time helps ensure that units are compatible. Mismatched units may work fine together temporarily but in the long term you may encounter serious problems. In addition, factory warranties may be void from using mismatched units. However, if your AC system is still covered under factory warranty but the outdoor unit needs replacement, you should see if the factory warranty can cover replacement. If it can, this may be one scenario where replacing just the outside unit makes sense.