The odds are quite high that an old heat only system has ducts that are too small to properly accommodate the air volume needed for air conditioning. In some cases you can improperly accommodate the air conditioner by putting an oversized blower and/or variable speed blower on old ducts. It's not ideal but it'll sometimes work. On rare occasion old ducts are big enough.
On average you'll want somewhere between 350 to 450 CFM per ton of AC, depending on how humid your area gets. Whatever solution you choose, the only way you'll know if your solution worked is to actually measure the final results. Measuring the output of the system seems so fundamental, yet it usually doesn't happen. I'd suggest that you make airflow measurement a part of whatever package you get.
If you are dealing with a reputable contractor, then they should be more than willing to explain to you their recommendations for their entire proposal including air distribution changes. Just because a system was heat only, does not in and of itself, dictate the ductwork be replaced. At the same time, because it was a heating only system, modifications may need to be made to add air conditioning to the system. These are all areas that you need to be discussing with your contractor. Geeesh. Just ask the guy! All the best, John.
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