If the cable housing is good just leave the housing in and remove the cable, that will make putting it back in easy. Also these are the annoying insert aero optional levers and though I didn't say it I had a feeling you wouldn't be able to get at the nut without removing the cable entirely since they are in the aero configuration. I have these in aero on one of my bikes (the cobalto levers to be exact). Anyway, yeah you may have to take the cable out completely. The hard part is putting it back in. Trying to keep the insert in place and getting the cable end fitted in the barrel thingy can be a real PITA. You have to keep constant tension on it. If it goes slack and then pull it tight to secure to the caliper the barrel often spins and the cable end comes out because of the geometry of the aero configuration. An awful design we tolerate for the aesthetics of campagnolo.