Drop in gas to heat pump conversion doesn’t work well due to duct sizing. Gas is much hotter so ducts are too small for the small delta T heat pumps produce
This is commonly cited, but in practice it often isn't as big of a limiter as you'd think. Many homes have drastically oversized furnaces, so if the ductwork is decently sized for that furnace then the much smaller heat pump actually ends up with adequate ductwork.
Also, many heat pumps are designed to be able to operate at high static pressures, so they can push air through relatively small ducts albeit at higher fan speeds.