I agree with the mobile tec
I had ducted Carrier Air V's.. No longer made.. I now have two different makes.. We replaced JUST the rooftop unit (Mechanical parts) on one and both the roof and ceiling gred on the other.. The one still uses the original Carrier contgrols (The newest one has "Local" controls)
IN many cases the ducting uints are common make to make and you can replace. IN some cases you can "Adapt" a roof top unit to the existing lower/control unit as well.. That's how we did my bedroom.