It's always nice to have two sets of wheels. Use the existing ones for unladen day rides and get another set for actual touring. You don't have to spend a lot of money but there is no gain to light wheels when a few lbs of racks and empty panniers totally erase the benefit of light wheels. Once you put 20lbs of dead weight on the rear wheel your simply getting closer to breaking spokes with zero "performance" gain in the light wheels. What you get with robust wheels is greater chance of wheels surviving abuse while riding or in the course of the bike being picked up and moved around in less than ideal conditions.