Replacing your furnace and air conditioner together, known as a "matched system," can have several advantages. It ensures compatibility between the two units, maximizing efficiency and performance. A matched system can also be more cost-effective in the long run as it reduces the risk of mismatched components, which can lead to inefficiency and costly repairs. Additionally, newer HVAC systems often incorporate advanced technology and higher efficiency ratings, which can result in energy savings and improved comfort. However, if one component is still in good condition and significantly newer than the other, you might consider replacing only the older unit to extend its life, but it's crucial to consult with an HVAC professional to evaluate your specific situation and make an informed decision.