Replacing anything piecemeal ultimately costs more for several good reasons:
When you replace pieces of your heating & cooling system it will always be limited to the performance of the most inefficient piece. Putting a high efficiency 16 or 18 SEER condenser on an old 12 or 13 SEER system will result in a system that operates closer to 12 or 13 SEER and rob you of valuable savings.
When you replace only sections of a system it will result in more frequent repairs, not only as older parts would normally wear out, but also because newer equipment operates at higher pressures which equipment ten years ago was not designed to handle and may put an added strain on older parts.
Replacing all parts of the system can save up to $2,000.00 in labor and materials because you don’t have to repeat the labor for each piece.
When replacing your complete system, you end up with better warranties covering labor, materials, and in many cases, up to double the warranty you would have if you did it piecemeal. There is no question regarding what was done, did the old part cause the new part to fail, or if the materials used were new or preexisting.
The problem is that most furnaces last longer than most air conditioners. The average lifespan of an air conditioner is about 12-18 years whereas the average lifespan of a furnace is about 15-30 years. The biggest thing to remember should be that an old furnace, air conditioner, heat pump, or coil, is kind of like that beat-up old 1967 Pontiac Bonneville that got at best 11 miles to the gallon. You stopped driving it years ago because it cost a fortune to operate.
In certain rare instances, it may more practical to reuse parts of a system, but only when a professional can prove that the piece in question is not approaching the end of its life and that it will not reduce the system performance significantly. Keep in mind that you will still be running an unmatched system and it may sacrifice some of the air conditioner’s efficiency. Replacing one piece and not the other may also result in premature failure.