Generally, yes. Most units are designed in matching pairs—the indoor and the outdoor—so most likely, you’ll need a new indoor system to complement your new outdoor unit. Having a matching set ensures efficiency, saving you money in the long run. You wouldn’t want to pair an old unit with a new one, as many technological advancements have taken place in recent years. Even if they work together, you are not getting the best performance possible. Replacing them both will also save you from having unnecessary work done on the newer unit when the older one is acting from years of wear and tear.