Has anyone done or seen soffit or chase retrofit inside home envelope from flex duct in the attic? In other words, move ducts indoors.

I want to get rid of my 20-year-old falling-apart money-hungry flex ducts that are in the attic. Read that it is more efficient inside. My Jim Walter home is more like a double wide mobile home about 1200 sq ft single story ranch, raised foundation crawl space, no basement, central air with air handler inside.