The new unit gets the attention. The ducts decide how much of its work reaches the rooms. Before the equipment order is final, walk the duct system the way the installer will, and write down what you find.
Start at the Return
Find every return grille and follow each one back to the air handler or furnace. Note which rooms have a return and which do not. A closed bedroom door with no return path can starve that room of air and push pressure into the rest of the house. Ask the installer how the return side is planned, and whether any added return is part of the scope.
Look at the Supply Runs
Walk the supply trunk and its branches to the main floor, the upstairs rooms, and any finished space over a garage or porch. Check for crushed flex duct, sagging runs, and joints held with tape that has dried out. Ask whether the plan includes sealing the joints and checking total airflow, not only the size of the new box.
Check the Attic, Crawlspace, and Basement Runs
Ducts that run through unconditioned space often need the most attention. Look at the insulation on the duct, the condition of the wrap, and any section that passes through a space that gets hot or cold. Write down what you can see. Anything hidden behind finished walls is a question to put to the installer before the work starts, not a surprise for install day.
Match the Plan to the Duct Walk
Before you approve a quote, ask the installer these questions in writing: whether the existing ducts were measured or only assumed, whether any duct changes are priced separately from the equipment, which parts of the duct system will be sealed or replaced, and who decides if the existing system cannot carry the new equipment's airflow. Put the answers from each installer side by side. Vague answers are a reason to ask again, not a reason to proceed.
For the equipment side of the decision, the Gas Furnace planning page covers service life and when to plan a replacement, and the Central Air Conditioner page does the same for cooling. Use the duct walk to confirm that whichever unit you choose has a sound air path to work with.