Unless you're wanting to get ALL of the old fluid out for the purpose of switching to some sort of synthetic alternative, there's no need to do a full flush. I did a pan drop and filter change at about 50k miles and plan to do just a pan drop (actually drain plug on my aftermarket pan) at 60k and then a pan drop and filter change every 60k thereafter. The main reason I will be doing regular changes rather than subscribing to the OEM lifetime fill suggestion is that the transmission contains an expensive valve body with many very sensitive electronic solenoids. Over time, the small metallic particles circulating in your fluid (filter doesn't catch everything) will collect on the electrical terminals of those solenoids which can potentially necessitate the need to replace them prematurely. Replacing the amount of fluid equivalent to a pan drop should provide enough of a fluid swap and refresh that it illeviates the potential for the shift selenoids to short out due to contamination on the electrical contacts. Otherwise, the 6r80 appears to be a fairly reliable transmission as long as you do a little preventative maintenance.