Can I keep my existing ductwork when replacing a furnace with a heat pump and air handler?
Definitely maybe...  Sorry but it depends on whether or not the existing ducting will work with the new unit.  So you should take that into consideration when selecting a new air handler.  Many units are similar but not identical and so you may have to have an "adapter" fabricated to mate up the new unit with the old ducting.

Why do you think the old unit is oversized?  It would be unusual to oversize like that because a larger unit is more expensive and a waste of money if it's larger than the needed size.  You should have an HVAC professional evaluate your home and make a size recommendation.