The answer is most likely yes, and here are the reasons why.
First of all, all air conditioner and heat pump outdoor units are specifically designed to work with matched indoor units for optimum efficiency and performance. The result of this matched system is a coordinated, top-performance team that ensures dependability and efficiency. Air conditioner and heat pump outdoor units may "work" with indoor units other than those for which they have been specifically designed; however, the result is a definite compromise in system performance.
In addition, through the years, indoor blower coil units have undergone numerous design advances — especially in the areas of air handling performance, filtering efficiency and operating sound levels. A new outdoor unit will also include the latest design advances.
The cooling and/or heating efficiency ratings that are advertised for an air conditioner or heat pump are based on their performance as matched systems. While changing only the outdoor unit may result in improved efficiency compared to the old system, it will not be as efficient as it was designed to be and your savings will be lower than with a matched system.
If an air conditioner or heat pump outdoor unit is 10 years old and needs to be replaced, the indoor unit is just as old and has been subjected to the same amount of wear and tear. Replacing both units means you won't have to replace the indoor unit in a short time...you'll have years of service from both units.
Last, but not least, a new unit also gives you a new product warranty. Replacing the indoor unit at the same time as the outdoor unit results in peace of mind, knowing the new warranty covers your entire system. At first glance, replacing only an air conditioner or heat pump outdoor unit may appear to be a bargain. But when you consider the lower cooling and heating efficiencies, decreased reliability and high cost of ownership that results, it's not such a bargain. Replacing an entire system costs more, but you get more efficiency, reliability and comfort.