Skydel is indeed flexible, but horribly manual-labour intensive. I too helped a re-role a time or two in my short time on exchange.
"Hey loady, need some help? Or do you want it done right?"
Another nice feature of the -4 floor is those 463L pallets that slide on also slide into the C17 w/o disassembling or floor loading or any of the other nonsense skydel required.
Strat air to tac air to user...463L pallets work well & have been doing so in USAF since Viet Nam - one common pallet system for entire AT fleet.
As for altitude reporting on the J - I'm just guessing here, but doubt the actual altimeters themselves have anything to do with it. Would likely suspect all is converted from an ADT or three & RVSM is principally a money/certification issue - could've been done by Lockheed (for a fee, no doubt).