replacing ductwork in a mobile home with round duct?

I know that most mobile homes use a single trunk duct that is 14" x 4" and all use a special furnace due to that size being used. , how easy is that to replace? I found out most of my duct work in my 1979 mobile home is bad , for some reason most of it is completely crushed and has a whole bunch of holes it it. Its in very good shape except for the duct work.

I want to replace it with round duct since I’ve been told that round duct is more efficient , the equivalent round duct sizing is about 8" to 9" , I was also told I could install slightly larger duct work and that would fix the high static issue with the small duct work , which is the reason why mobile homes use a special furnace with a higher RPM blower.