The house I'm living now has a boiler for heat and condensador for cooling, so I have both systems, I want to get rid of radiator because the use too much space nad also they are chipng, but my problem is because my house was not built with duct we run the duc work for the second floolr trhough the crwal space between the roof and ceiling using flexible duct, resulting in the vents to be in the celing in the second floor. My big cocern is if change my boiler for a furnace is my secnd floor going to be cold in th winter because of the vents in the celing ( we al know the hot air raises and cold air drops so I;m not sure if in the winter I'm goint to be cold in the second floor.
Tank you.

What do you reconmend me.

It really depends on the size and lengths of ducts. If well designed the velocity from the register will push the warm air to the floor and Eddie currents of air will mix the air well enough.