Bob, We also have roof air units w/heat pumps (2). In addition to the outlets directly below the units, there is duct work with outlets throughout the coach roof...the outlets below the units ARE noisy so I understand the desire to 'quieten' things down a bit. However, I would think the retro would be highly cost prohibitive.
Unless the original manufacturer does the mod, special duct work would have to be built and fitted to tie into existing overhead duct work from the new basement units(and the placement or mounting of these units may be problematic as well...room?), up the sides of the coach which would require either pulling inside/outside and ceiling paneling or installing exposed duct work on the interior/exterior...NAH. All the associated wiring to configure in and around existing plumbing and wiring, sprayed in foam insulation... GADS!
I really do not care for the noise these units put out either, but my plan (based on my current rate of hearing loss) is to wait a couple of years and all that blow'n will not be NEAR as annoying!
Other than the above, give Gulfstream a call to see if your plan falls into the realm of possibility. If however, you follow my plan, please send me 10% of your cost savings...I need fuel and I should be able to crisscross the Rockies 'bout six times on what you could send!
Thanks and good luck with the plan. Bob