central ac air handler

It depends on what changes are going to be made. They need to be sized correctly or you may get poor results and damaged equipment.

Seems to me that a GOOD hvac man can put in a system with a variable speed fan in the air handler and use the existing duct work. It may make a little more noise than with the proper ducts but you will get most of the efficiency you want.

It would take a GROSS mis-design of the duct work to "damage" the system.

I agree. I should have taken my answer further and indicated that the specific situation needs to be examined by a GOOD HVAC professional. Of course it does not take much of a sizing error to cause poor performance.

I have been looking into replacing my a/c unit. Garage ceiling mounted air handler and compressor outside. Noisy as,,,,,, well you get the idea. Every site and every contractor I have checked, 20 odd. Says the same thing, the compressor, high seer MUST be matched to the air handler. Which means if your changing from a 10 to a 12 why bother. If like me changing from a 10 to a 14 why would I not get the max out of the compressor. Sort of like having a race car and putting low end tires on it, then expecting it to handle correctly

I should think the O.P. could have heat gain/loss calculation and duct design for the final arrangement and size of things, then live with a possibly mis sized AHU for a year. TB

Can you clarify? When I see "compressor" and "winter" I think heat pump, not straight A/C. If so, can you run emergency heat, portable electric heaters, or perhaps kerosene next winter? Basically, hold off until you can put a complete new system in. Otherwise, you'll become well-acquainted with the concept of "time and material." Additionally, it's usually a really bad idea to run an HVAC system during construction. Better to trash the old system than a new one.