The case of duct work not being sufficient for a oil/gas -> heat pump conversion is pretty common. A heat pump, especially one of the conventional split types (

The case of duct work not being sufficient for a oil/gas -> heat pump conversion is pretty common. A heat pump, especially one of the conventional split types (non-minisplit) generally wants 300-400 CFM of airflow per ton. So, a 3 ton unit is happiest moving around 1200 CFM of air. Lower can cause issues with ice-up in AC mode or high pressure shutdown in heat mode. There are rules of thumb for duct work that are pretty easy to use to estimate. A 6" round duct, for example, is designed for about 80 CFM (not counting any restrictions on the register/grille.) That 3 ton system is going to need 15 'takeoffs' of 6" duct to handle that air. Adding more duct work can be expensive if the majority of it isn't in unfinished locations like a basement or attic.