There are only a few unit upgrades in LR are not available in DR, such as pavasses for crossbowmen/heavy missles. Other than that the unit entries in LR are the same in DR. I play my LR warband in DR all the time, and do quite well.
As an aside, the upgrades to units in both rules may be a reason to get both books. That way you would have more options to fully realize your vision of a unit. And don't forget Dan Mersey's rules for Dark Age LR! Even more options to use!