I think you could keep the 6.8.4 OD software. There was the "re-release" of 6.8.9/6.8.11MM that was offered with the new 6.8.5 IIRC. However, many people including myself, did not think it was quite the same. The newest tune seems to be working just fine and mileage is pretty good.
'10 2500 Laramie CCSB BBCP 4X4 68RFE, 285/70-17 Dueler Revo 2's, Black Maxx w/ MCC and OverDrive, 4" TBE w/ 5" tip, Airaid Intake w/ SynthaMax filter, AMP Power Steps.
The dealer said my truck needed new sparkplugs 6>8