I would think the 18 years of service would be a good case for it not being a big problem.
The only thing would be is if the 2 1/2 ton air handler is a dedicated one size only unit. The effect would be a loss of efficiency..... would cost you more to operate verses a properly sized pair. As long as the system was setup to deal with the potential imbalance, no harm, just a foul so to speak.
"Some" air handlers were and still are designed and built to be able to be a match for more than one size condenser. Sometimes, but not always, this is identified in the model number.