Upper or Lower Adjustable Control Arms?

Hello, I have heard a couple different theories and would like to get a consensus here. I have a 3" lift with stock control arms; I am going to a 4.5" lift and need to know which arms NEED to be replaced. Do they both have to be replaced or is it acceptable to replace just one set at a time? I will likely continue to run 31's on the trail and 235's on the street as regearing is not an option right now. It may look kinda retarded but the MPG on my 80 mile daily commute adds up, and with winter coming I don't think the 31's have much to offer as they are a M/T- although the 235 BFG AT's like to break loose. plus I figure I'll save a little undue stress on my transmission and rear end by not running the 31's with 3.55's on that long of a commute.
How far off base am I on my theory?
Thanks for the help.