Switch from floor vents in slab to ceiling vents in attic?
 

Can I switch over (with existing furnace) from floor vents in a 1950's slab, to vents/ducts that run through the attic and out the ceilings??? If so, how difficult is this process, is it extremely expensive??

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Contact a heat and a/c company to give you solutions. I would get the vents out of the floor, to much contaminationand deterioration to the ductwork takes place. And if you have rodents...they live in there.
Go for the attic route with insulated ductwork. Flex duct will be cheaper, but does not do the same quality job.