The recommendations on changing a DF bike chain may be a CYA for the manufacturer. You can't stand up on a recumbent and put all of your weight on the cranks as you can on a DF. Your body weight exerts a heck of a lot of force on the chain and if it breaks you can be in real trouble. At the least you may be stranded. I have three chains that have over 5,000 miles on them and don't seem to need replacement yet. One has at least 7,500 miles (a trike).